Best harness guide I've seen for these cars.Wire Guide for your Grand Prix - Audio System
EDIT:
the outputs from the factory bose HU are low-level (I believe 5V) in comparison to the 12V the non-bose system and aftermarket head units put out. Way to make everything play nicely and not need to worry about blowing that hard-to-find bose amp, the best solution is to use the RCA output from the aftermarket HU. (I ran RCA cables to the rear of the car and spliced female RCA adapters from RadioShack into the harness on the amplifier)
