2004 Sierra Delphi Radio CD Cassette player sound intermittant and no door chimes

I have a 2004 GMC Sierra with a snarky audio system. The left door speakers and rear speakers and all chimes do not work at all. The rest of the sound is intermittantly working. It either works normally or is at about 1/8 volume. I tried a freinds head unit that worked fine in his truck and before it locked it worked the same as mine did. I assumed that I have an amplifier problem but I cant figure out if I even have an amp installed. Other threads mention amps for Bose systems but none for the Delphi stock audio system.
Does anyone actually know if an amp is installed or what the problem could be.
Thanks