The Ultimate DIY Guide: Your BRZ tS LH Taillight Install and Critical Leak Test

You are about to save hundreds of dollars in dealership labor costs and protect your prized Subaru BRZ tS from a common and insidious problem. This comprehensive guide will walk you through every step of the BRZ tS LH taillight install, but more importantly, it will show you how to perform a non-negotiable water leak test. A shocking number of BRZ, Toyota 86, and Scion FR-S owners discover water sloshing in their trunk, leading to mildew, electrical gremlins, and rust, all because of a simple, poorly sealed taillight. By following this tutorial, you will not only complete the installation correctly but also ensure your trunk stays dry for years to come.

Why This Guide is Essential for Every BRZ/86/FR-S Owner

If you have ever found a puddle in your spare tire well or noticed a musty smell emanating from the trunk, you are not alone. This is a well-documented issue across the Subaru BRZ, Toyota 86, and Scion FR-S platform. The factory taillight gasket can become compromised over time, or it may not seat perfectly during a replacement. The consequence is water channeling directly from the rear of the car into your trunk compartment. This is not a minor inconvenience. It can lead to:

• Corrosion of the trunk floor and frame components.
• Short circuits in the taillight wiring, causing bulb failures or fuse blows.
• Mold and mildew growth, creating a health hazard and a persistent odor.
• Damage to any items you store in the trunk.

This guide positions you to tackle the entire job with confidence, transforming a potential headache into a straightforward Saturday morning project.

Gathering Your Tools and Parts

Before you lay a finger on your car, gather everything you need. There is nothing more frustrating than being halfway through a job and realizing you are missing a crucial tool. For this Subaru BRZ taillight replacement, you do not need a garage full of exotic tools.

Essential Tools

• Trim Removal Tool Set (Plastic pry tools to prevent scratches)
• #2 Phillips Screwdriver
• 10mm Socket with a Ratchet and Short Extension
• Flashlight or Work Light
• Microfiber Towel
• Garden Hose or Large Spray Bottle

Parts and Materials

• Replacement LH Driver's Side Taillight: Ensure it is the correct part for your model year. The BRZ tS taillight may have slight differences from the standard BRZ, such as a clear versus smoked lens, so verify compatibility. You can source a new OEM unit from a supplier like Genuine Subie Parts.
• New Taillight Gasket: This is highly recommended. Do not reuse the old, potentially compressed gasket. A new seal is cheap insurance. This is the key to preventing trunk water damage.
• Dielectric Grease (for electrical connector)

Step 1: Safe Removal of the Old Left-Hand Taillight

This process is logical and gentle. The goal is to remove the interior trunk trim without breaking any clips, then unbolt the light itself.

Step 1.1: Gaining Access to the Taillight Bolts

Open your trunk and locate the large interior trim panel on the driver's side. You will see a small rectangular access cover; you can pop this off with a flat-head screwdriver or your trim tool, but for a full BRZ tS LH taillight install, it is best to remove the entire side panel for better visibility and access.

1. Remove the Cargo Carpet: Pull the trunk carpeting away from the side panel.
2. Locate and Remove Plastic Clips: You will see several plastic pop clips holding the panel in place. Use your trim removal tool to gently pry the center pin of each clip up about a quarter-inch, then you can pull the entire clip out. Do not just yank on the panel, as this can break the clips or the panel itself.
3. Detach the Panel: Once all clips are removed, carefully pull the panel away from the body of the car. It may still be held by a plastic hook at the top. Gently maneuver it free and set it aside.

You should now have a clear view of the back of the taillight housing, with its wiring connector and three 10mm bolts.

Step 1.2: Disconnecting and Unbolting the Taillight

1. Disconnect the Electrical Connector: Loc the plastic wiring harness connector plugged into the taillight. There will be a small tab you need to press down to release it. Firmly press the tab and pull the connector straight off. If it is stubborn, a gentle wiggling motion can help. A small dab of dielectric grease on the new connector's terminals during reinstallation is a great practice for corrosion prevention.
2. Remove the Three 10mm Bolts: Using your 10mm socket and ratchet, remove the three bolts that secure the taillight to the vehicle's body. Keep these bolts safe.

Step 1.3: Removing the Taillight from the Exterior

With the bolts removed, the taillight is now free. Grip the assembly firmly and pull it straight back towards the rear of the car. It might require a slight wiggle to break the seal of the old gasket. Be careful not to use the lens as a lever, as it is made of plastic and can crack. Pull from the housing itself.

Once the light is out, remove the old gasket from the vehicle's body or the back of the taillight. This is the perfect time to inspect it. Look for cracks, brittleness, or areas where it is permanently flattened. These are all symptoms of a failing taillight seal.

Step 2: Proper Installation of the New Taillight

This is where precision matters. A rushed installation is the primary cause of future leaks.

1. Clean the Mounting Surface: Using a microfiber towel, thoroughly clean the body of the car where the taillight gasket will seat. Remove any old dirt, debris, or remnants of the previous gasket. A clean surface is essential for a good seal.
2. Fit the New Gasket: Take your new taillight gasket and carefully fit it onto the new (or your original) taillight assembly. Ensure it is seated evenly all the way around in its channel.
3. Position and Install the Taillight: Carefully maneuver the new taillight into position from the outside. Align the three mounting studs with the holes in the car's body. Push the taillight straight in until the gasket makes full contact with the body. You should feel a slight compression.
4. Reinstall the 10mm Bolts: Hand-tighten the three 10mm bolts first to ensure proper threading. Then, using your ratchet, tighten them down. This brings up a common question: How tight should the taillight bolts be? The answer is snug, but not Hercules-tight. Overtightening can warp the taillight housing, crack the lens, or over-compress the gasket, creating a leak. A good rule of thumb is to tighten until the bolt head is just seated and then give it a little more, about a quarter-turn. The goal is firm contact without brute force.
5. Reconnect the Electrical Connector: Push the wiring harness connector back onto the plug on the taillight until you hear a definitive click. Give it a gentle tug to confirm it is locked in place.

Step 3: The Non-Negotiable Water Leak Test

Do not skip this step. Do not assume it is sealed. Verifying your work is what separates a professional-level job from a potential disaster. This is the core of how to test taillight for leaks.

Preparing for the Test

You have two main options for simulating rain: a garden hose with a shower-spray nozzle or a large spray bottle filled with water. The hose is more realistic. Leave the interior trunk trim panel off so you can see the back of the taillight from inside the car.

Executing the Test

1. Have a friend stand inside the trunk with a flashlight, focused on the area where the taillight meets the body of the car.
2. From the outside, gently spray water over the newly installed left taillight and the seam where it meets the car's body. Do not use a high-pressure jet stream, as this is not representative of normal rain and can force water past seals that would otherwise be fine. Use a gentle, shower-like spray.
3. Spray for a solid two to three minutes, moving the water stream around the entire perimeter of the taillight.
4. Your helper inside the trunk should be looking closely for any trickle or drip of water coming past the gasket and into the trunk space. Even a single drop is a failure.

What to Do If You Find a Leak

Do not panic. If water is getting in, it means the seal is not perfect.

• Re-check Bolt Tightness: Ensure the bolts are snug. An overly loose bolt is a common culprit.
• Re-seat the Gasket: Remove the taillight and check if the gasket is perfectly seated in its channel. Sometimes it can get pinched or folded during installation. Reseat it carefully.
• Inspect for Debris: Look for any small bits of debris on the car's body or the gasket that could be preventing a full seal.
• The Nuclear Option Sealant: If the leak persists, you may need to add a small amount of sealant. A common question is: Can I use a generic sealant instead of the OEM gasket? The answer is, you should not rely solely on sealant, but it can be used as a supplementary bead. Use a non-permanent, automotive-grade butyl rubber sealant (like 3M Strip Calk). Apply a very thin bead to the body side of the gasket, then reinstall the taillight. This creates a secondary seal. Avoid silicone-based sealants as they can be messy and difficult to remove later.

Repeat the water leak test until you achieve a perfectly dry result. Once confirmed, you can reinstall the interior trunk trim panel by snapping the clips back into place and repositioning the carpet.

Frequently Asked Questions (FAQ)

Final Thoughts and Recommended Resources

Completing this BRZ tS LH taillight install and leak test is a rewarding project that enhances your connection to your car and builds valuable DIY skills. You have not only replaced a part but have also proactively safeguarded your vehicle against a very common issue. The peace of mind that comes from knowing the job was done correctly is priceless.

For all your OEM part needs, from taillight gaskets to full assemblies, check out our trusted partner, Genuine Subie Parts. They offer a wide catalog of components to keep your BRZ, WRX, STI, or other Subaru models in perfect condition. For more detailed guides on Subaru maintenance and upgrades, like our article on Symptoms of Bad Shocks and Struts, explore our blog.