There is also a difference in impedence between line level and speaker inputs. while not readily noticable, an impedence mismatch can and will lead to distortion.

Mike - what kind of mounting kit or modifications are needed to put in a decent receiver? I'm looking at several units from Crutchfield. I also noticed the radio gets its power on signal from the GM LAN signal, I guess I could get that from the retained power from the power windows (that's what I'm using to power my XM receiver)

 

...but aren't the inputs to the amp on the pioneer system low level inputs vice speaker level? The diagram has them as low level and they do go into the amplifier right? I am only jumping in because I am getting ready to upgrade my system when I get my car back. I like the stock head unit and I already have the amps and an old MTX crossover out of my old truck. I just want to order a nice 8" sub and some speakers. Looking at alpine for the doors and polk for the sub.