Here is the info from the Jan/Feb 2003 Star (US mercedes club publication)Page 85

The question was on how to get an ASR off switch on models that dont have an ESP off switch. Their answer;

"Some of our advertisers sell a switch to defeat traction control for this purpose. See the Performance Products catalogue or web site () The switch is not difficult to wire yourself, and this will let you slide the back end all over the place.
Another way to accomplish this is to figure out which of the two co-located switches in the brake light switch signals the traction control system that you are engaging the brake pedal, then route a defeat switch to engage that signal. The brake light switch is normally open, and the other switch is normally closed. Wire the normally closed circut so that it is normally open, and you have defeated the traction control. The traction control disengages when it gets an open signal from that portion of the brake light switch, so you are telling it you are braking. Simple, eh? You can learn an amazing amount of information from factory manuals. Of course this is for off-road use only..."