Definately sounds like a wiring issue. From the description I'd suspect a poor ground connection, causing it to float upwards in voltage instead of staying down at 0V when other stuff that uses the same ground point pass current through it.

First thing I'd do is to make a new ground connection for the empeg, to a solid piece of metal (and make sure the metal part actually is connected to the rest of the car, as I recall someone who had grounded to a metal piece in the dash, to find that it was mounted all in plastic... )

/Michael