WRX STI Wagon Rear Seats Install: The Ultimate GD/GG Chassis Compatibility Guide

For countless Subaru WRX wagon owners, the pursuit of the perfect STI interior upgrade hits a frustrating roadblock when they discover the rear seats aren't a direct swap. While front seats typically bolt right in, the rear seats present a significant challenge due to the wagon's narrower body compared to the STI sedan. This comprehensive guide demystifies the entire process, providing a detailed roadmap for successfully completing the WRX STI wagon rear seats install that has perplexed many enthusiasts. We'll cover part compatibility, necessary modifications, and professional tips to achieve that coveted full STI interior transformation.

Understanding GD/GG Chassis Compatibility

The fundamental issue with the STI rear seat fit wagon scenario comes down to body dimensions. While sharing the same platform, the Impreza WRX wagon (GG chassis) is approximately 2.5 inches narrower than the WRX STI sedan (GD chassis). This difference is most pronounced in the rear seat area, specifically in the width between the seat mounting points and the placement of the seat belt anchors.

This doesn't mean the swap is impossible. It means you need to understand what works and what requires modification. The upper seatback from a USDM STI sedan is typically too wide to fit between the wagon's C-pillars without modification to the seat brackets. The lower seat cushion also requires bracket adjustments to properly secure it to the wagon floorpan. Many enthusiasts have successfully completed this Subaru WRX wagon rear seat conversion through careful measurement and strategic modifications.

Essential Tools and Parts Checklist

Before beginning your GG wagon STI interior transformation, gather these essential tools and parts to ensure a smooth installation process.

Tools Required:

• Socket set (10mm, 12mm, 14mm sockets essential)
• Trim removal tools
• Angle grinder with cutting and grinding discs
• Welder (MIG preferred) or access to one
• Measuring tape and marker
• Center punch and hammer
• Drill with various bits
• Safety glasses and gloves

Parts Required:

• Complete USDM STI sedan rear seat set (upper and lower)
• Extra metal for bracket modification (if needed)
• High-strength epoxy (as backup to welding)
• Rust-inhibiting primer and paint

When sourcing your STI seats, look for quality units from reputable suppliers. The condition of the brackets you'll be modifying is particularly important, as corroded or damaged metal will complicate the process.

Removing Your Stock Wagon Seats

Begin your WRX wagon rear seat mod by properly removing the factory seats. Start with the lower cushion, which is typically held in place by two clips at the front edge. Pull upward firmly but evenly to release it from these clips. Once free, you can access the bolts securing the seatback.

The seatback is secured by bolts at the bottom and typically through the pass-through area. You'll also need to remove plastic trim pieces along the sides and seat belt anchors. Use trim tools to avoid damaging these pieces, as you'll need to reinstall them later. Label all bolts and hardware as you remove them to simplify reassembly.

With the seats removed, take this opportunity to thoroughly clean the interior area. You'll have access to spaces that are normally difficult to reach, and it's much easier to do this before installing the new seats.

Required Bracket and Striker Modifications

This is the most critical phase of the WRX wagon STI seat swap. The modifications required are substantial but manageable with the right tools and patience.

Upper Seatback Modifications:

The STI sedan upper seatback is approximately 2-3 inches too wide for the wagon body. You'll need to remove approximately 1-1.5 inches from each side bracket. Carefully measure and mark the cutting lines before using your angle grinder. The goal is to narrow the bracket while maintaining its structural integrity.

After cutting, test fit the seatback in the wagon. You may need to make additional adjustments for proper clearance. Once you've achieved the proper fit, weld the brackets back together at their new width. If you don't have access to a welder, a high-strength automotive epoxy can work, but welding is strongly recommended for safety.

Lower Cushion Modifications:

The lower cushion requires similar bracket adjustments. The mounting points on the wagon floorpan are closer together than on the sedan. You'll need to either modify the existing STI brackets or create custom mounting solutions. Some enthusiasts have successfully adapted the wagon brackets to work with the STI cushion, which can simplify the process.

After completing all modifications, thoroughly clean the metal surfaces and apply rust-inhibiting primer and paint to protect your work. This extra step will prevent corrosion and ensure the longevity of your modifications.

STI Seat Installation Process

With your modified seats ready, begin the installation process. Start with the lower cushion, ensuring it sits flush and level on the wagon floorpan. You may need to test fit multiple times to achieve perfect alignment. Once positioned correctly, secure it using the appropriate hardware.

Next, install the upper seatback. This is typically the more challenging part due to the need to align with the newly positioned lower cushion and the body-mounted strikers. You may need to adjust the striker positions on the wagon's C-pillars to ensure proper engagement with the seatback latches.

Finally, reinstall all trim pieces and ensure all seat functions operate correctly. Test the fold-down mechanism multiple times to verify smooth operation and secure locking. The STI rear seat pass-through should function similarly to how it did in the sedan, allowing access to the trunk area.

Safety Considerations and Final Checks

When performing any Subaru rear seat bracket modification, safety must be your primary concern. Modified seat brackets represent a potential point of failure in a collision. Your modifications must maintain at least the same structural integrity as the original design.

After installation, conduct thorough testing. Check that all latches engage securely and that there's no excessive movement in the seat assembly. The seat belts should function correctly and retract properly. Have a helper vigorously shake the installed seats to check for any unusual movement or sounds that might indicate insufficient securing.

Consider having your work inspected by a professional if you have any doubts about the safety or integrity of your modifications. The peace of mind is worth the additional investment, especially when it comes to components that are critical for passenger safety.

Frequently Asked Questions