Make certain you have a clean connection at your amp ground. The paint should be removed with a dremel or file to make certain you are getting good ground contact.

The noise may have less to do with your amplifier ground and more to do with your signal source. How are you getting the signal to the amps? I had horrible engine whine in my previous vehicle. I had to try a number of solutions before solving. Line drivers, different rca cables, different routing, etc. My amp ground was never the source.

__________________
2004 BMW X5 3.0i - Mine
2011 BMW 328i xDrive - Hers
2002 BMW 325xi - waiting for a new owner