...related to the rear channel feeds are properly connected and isolated from feedback and interference, and everything is grounded properly, then it's probably a good bet that it's the amp itself. One thing you should check if it hasn't been done yet is to swap the front/rear lines to the amp, just to make sure that it's not the rear line-out in the h/u that's causing the problem.
In either case, if the gear's new and proven defective you should have them REPLACED and not repaired, just as long as you didn't sit on the stuff prior to install and allowed the return period to expire.
I just had an Alpine PDX-4.100 amplifier installed along with an Alpine IVA-W205 head unit, Type-X 6.5" components in the front, and Type-R 6x9s in the rear deck of my 2003 Infiniti G35 Coupe. Immediately upon getting into the car, I noticed there was a high pitched humming coming from the back 6x9s. It is not coming from the front speakers at all. The installer told me it is most likely a problem with the amplifier, and I'm just wondering if there is anything else I can try before I send it back for repair or replacement. It makes sense that it is the amplifier because it is constant and the volume never changes, and it is only in the back. It only stops when I turn the head unit off.