one thing i don't understand is if the issue is the poor ground to the RCA, why would the engine running (alt working) make any difference?

Explained here.

Quote:

    "When this happens, your stereo's audio output circuity suddenly becomes part of your car's electrical system. Every single thing in your car that interacts with the electrical system will be amplified by your stereo. The alternator, the spark plugs, the air conditioner, the headlights, the brake lights, the car's emissions-control computer, the stereo's electronics, the electric motor that spins the CD in your CD player (or spins the hard disk in your MP3 player), everything. All of these things will suddenly cause non-musical noise to come out of your speakers."
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Tony Fabris