[ischmitz]

Pin 3 from the 55-pin DME connector runs through the DME harness to pin 2 of the 14-pin connector under the driver's seat. From there it runs on to the DME relay socket Term 85b and turns on the 2nd stage (fuel pump, etc) when motion on the flywheel is detected. Interrupt that line somewhere and you have your immobilizer.

Personally, I'd stay away from the main power since you would need to control 20+ Amps and your unit might not be up to that task even if it has an internal relay. Further wrestling with 4mm wires requires some skills in soldering and if improperly done adds safety hazards (poor joints warm up and cause cable insulation melting, etc.)

Ingo