I decided to post an answer because no one else has taken a crack at this. You may want to post on the "Non-Tech" board. I have been impressed by the wealth of good answers on the radio topics...give them a try.
My original head unit went out and I am planning to replace it with an Eclipse unit that will plug right-in and not require any change in the original amp. (I am told that the top of the line Eclipse has it's own amp. and you bypass the original amp.).
Al35thSC seems to have a handle on all the stereo stuff.
Good Luck...
Ron