Alon- is correct, get the trailer wiring kit that senses but doesn't place a load on your taillights.

Placing a load on the tailights will cause problems with cruise control and the Lights-out monitor.

I installed the above kit in our 1988 wagon.

I tapped into the tailight connectors and powered it via the sunroof motor (yes I installed a fuse) positive (+) lead, instead of running a power lead all the way to the front of the car. Works like a charm, and no problems.

:-) neil
1988 360TE AMG
1993 500E