You originally said interior lights, which confused me. I see you are talking dash lights then.

Anyway I still think a wiring fault is your problem. My car has a dimmer control on the dash lights. I still use this signal for the dimmer control on the empeg. Even at the dimmest setting the voltage is still enough that the empeg can sense the headlights are on and it dims accordingly.

If the empeg is outputting a voltage into the dimmer circuit, it is potentially supply all your dash lights with power. This would be A Bad Thing (tm). What this would probably also mean is that even without your headlights on, the empeg would still light the dash lights. Is this the case?

I doubt the RCA into AUX is causing you grief.