[ischmitz]

To answer TWJ: I would run a thick short wire with an eye straight to the chasis. Use a self-tapping screw into the firewall or find a machine screw that can take the ground cable. When you wire it into the brown wiring that runs behind the instruments you could end up with poor GND and that can contribute to the hum. These brown wires use spade connectors that corrode over time. And they power the instrument lights and could have several 100mV above GND.

To answer Rob: Your car has Motronic so there isn't any CDI unit. Still, make sure your head unit has a perfect GND. Same advise as above: Use a thick wire with an eye and find a chasis screw. That is the best way to get GND

Ingo