sein: Thanks for the info. No problem buying cables; whatever will make it work.

The problem seems to be the ground wire I'm using is not strong enough (thick enough?) to do the job. Two head units and relay using the stock headunit ground wire.

To test, I'm tempted to run a jumper cable from the netagive terminal of the battery and clamp it to the ground wire of the three components. Do you think that will fry my audio components?

The jumper cable should be the most powerful ground wire ever. If it fixes the ground loop, then I just need to run a stronger ground to a bolt on the chasis (I've no idea how I'll work both the nut and bolt end) or have a negative line run from the battery (in which case, I might as well have a better power line run also).

And I was hoping this would be easy...