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Below are GM instructions, I have also included seat removal instructions for the EXT as called for in the instructions. also notice special precautions when disconnecting the BCM below.
Let me know if you have more questions
Rear Electrical Center or Junction Block Replacement
Caution: Unless directed otherwise, the ignition and start switch must be in the OFF or LOCK position, and all electrical loads must be OFF before servicing any electrical component. Disconnect the negative battery cable to prevent an electrical spark should a tool or equipment come in contact with an exposed electrical terminal. Failure to follow these precautions may result in personal injury and/or damage to the vehicle or its components.
Rear Seat Replacement (TrailBlazer EXT, Envoy XL)
Body Control Module Replacement
Slide the latch inboard until fully extended, approximately 40 mm (1.6 in).