Well your question of "where" the noise is coming from is apparent, and you meantioned it... inverter.
As to how to bandaid it, have you tried a bigger wire on your HU's ground?
Or trying the RCA to ground thing. I think you take 1 side and ground the shield the other you keep.
There may be more, those other than a GLI, those are the most popular options.
Also just to make sure, you do have your RCA's running in a different area than your power right?