Background Noise

I am installing a system into a 2005 Dodge Ram 2500 with an infinity sound system. I am wanting to keep the current head unit, but i am installing new speakers all powered by an aftermarket amp. My question is that there is always a hissing sound coming from the speakers at low levels, and i was wondering if anybody had any ideas on how to get rid of it? I have tried changing the ground, seperating the rca cable and the power cable, using the power supply and ground from a different vehicle, using rca cables coming from a different vehicle's head unit, hooking up an extra speaker directly to the amp to test to see if it was the wiring in the door, and using a different amp all to no avail. Any help would be greatly appreciated. Thanks.

i worked on a 1500 today same thing happened it turned out to be from the amp though but if you have the infinity system then your gonna need to bypass your amp, if you already ran new speaker wires from all the speakers to the amp the your still gonna need to unplug both harnesses from the side of your amp, the amp should be behind your glove compartment