Factory radio woe's

I know this isint a stereo forum, and I am not a kid trying to outbump the honda next to me at wal-mart on friday nights but I would like to replace my factory radio with something a little nicer.

My question: I have a 2002 standard yukon, no onstar. it does have the factory sub but that is all frills it has. The stereo shop is telling me that i have "Factory amps" and that I will have to pay a lot extra or buy an adapter to allow me to use an aftermarket radio.

Is this true?
If I do have factory amp(s) where is it (they) located? is it controlling just the sub or the sub and the door speakers as well?

From what i found out the amp is behind the glove box mounted to the firewall. It runs every thing doors, rear, and sub, go on line in a stereo form and ask someone for the wiring lay out to make your own adapter. A buddy just did this and it was a lot cheaper...but he is not happy with the sound so he is going to replace the amp and speakers.

Might get some help here.

12 Volt Forum (http://www.the12volt.com/installbay/default.asp?ForumID=0)