One thing you could try is unplugging all 4 RCAs and, with the empleg playing, touch the tip of one (or some) against the empeg casing. If this introduces noise then the test should prove to you it's not the empeg creating the noise. Unfortunately, the opposite effect (no noise) doesn't necessarily prove the empeg itself is at fault.

Also, have you checked for other unwanted earths in your wiring eg. speaker cables? I expect you've already done this; try running the empeg on the seat to rule out any accidental earthing taking place when it's installed in the dash.