Okay, so via the process of elimination you have determined that the cause is not internal with the amplifier. Continue to use the same logic to isolate each upstream component in the signal path.
If you tried wiring direct then I assume you have the noise without the key engaged?
Do you have the noise with all sources, ie; CD, radio, ipod/phone/aux?
If it is exclusive to radio then you could have a disturbance in the garage such as lighting.
If it is on all sources then you have to separate the HU from RCAs.
Make sure that all RCA cable ends are well seated with a fair grip. Try gently moving around the RCA cables at both ends. Use a substitute RCA cable.
Besides the HU harness ground, try grounding the HU chassis.
No meaningful diagnosis should take place without first verifying that you have a healthy voltage.
Also, check the voltage across the amplifier primary terminals with the amplifier under load.
Any action that creates a change in status should automatically narrow your focus.