One thing to check: disconnect the audio cables from the sled, so that there is nothing but power connected to the empeg. Using a multimeter, check that the empeg's case (when isolated from other metalwork in the car) is grounded to the car body.

We've had people who have had bad ground connections, and their empeg has been getting ground for power from the amp output audio shields. This causes noise and is very bad in general....

Hugo