Originally Posted by quality_sound
Ok, before anyone jumps in and says to check your grounds or any other such nonsense you need to see where the problem is exactly. Wife your front speakers are fine it's probably not the amp but we're going to make sure.
First, swap front and rear RCAs. If the noise moves to the front it's the rcas or the integration adapter. If it doesn't, swap the RCAs back. And swap the front and rear speaker cables. If the noise moves it's either the amp, rear speakers, or rear speaker wires. To find out which, connect a known good speaker and see if there is noise. No noise means speaker or wire. Noise means bad amp. If there's noise with the test speaker run a new speaker wire to one rear at a time and see which makes noise. If nothing on either, it's the wire. If only on speaker, replace that speaker.
When I was experimenting and I plugged the factory balanced differential signal into a component that used common ground input I got this exact effect. No whine in the front but whine in the rear speakers even though the fronts and the underseats were the only think hooked up to the amp. The rears were running of the factory hi-fi amp.