I believe you are right. I talked to the tech who installed and replaced my old infininity stereo in my 97 (with a new radio/Cd player) and he mentioned the speaker amps in the door where the front speakers are. I don't know how its wired tho.I believe that if your truck has the Infinity system that the speakers have amps built in. If you bought the deck on line contact them, I know Cruchfield will, they might give you instructions on how to fix it.
Floyd