Audio Systems: Amp probs., gmc suburban, gm trucks

Question
1999 GMC Suburban. This vehicle has two speakers in each of the front doors, One speaker each second doors, and two rear speakers in the roof. When operatio=ng the stereo with the engine off, everything works fine. If the engine is running, any volume at all causes the second door speakers and the rear speakers to shut down (amp shuts off?). Sometimes there will be some noises in the speakers at this point, such as squeels or crackles. Also, there is motor noise in the speakers before shut down(high pitched noise associated with the speed of the engine). Any ideas? I would like to know the location of the factory amp as well. Thank you!!  

Answer
I think the auxillary amp in under the driver's seat. Most GM trucks have noise issues because the dash is not grounded properly. Run a large (10G) ground wire from the battery negative post to the bottom of the metal dash frame. I measured my own once before this mod, and there was 40 ohms from the battery post to the metal dash frame. There SHOULD have been less than 1 ohm. After this has been done, get back to me.