Sendero, have you given up on the chassis to chassis ground approach? Although it's a little more challenging to get the strong ground needed, it makes for a cleaner install.

If you go with the inverter, I'd recommend using a relay driven by the switched power output of the Becker to control power to the inverter. This will keep it from drawing current from the battery when the car is turned off.

Here's the circuit I used:



This circuit virtually floats the power and ground of the Empeg. The only solid electrical connection to the car is through the audio cable. So you actually WANT to keep the Empeg chassis electrically isolated from the car chassis.

The top relay contact ensures standby power is applied to the Empeg (to keep the clock ticking et al). The top diode blocks power from being forced into the wrong side of the inverter, and the bottom diode just makes sure that the orange voltage isn't higher than the yellow.