EVERY TIME this has come up, the issue was no power to the ABS module. THis is caused by a failure of the RCM (Relay Control Module).

GO order a relay control module for a 2003 Mercedes SLK320. (Or order one for a "Crossfire" if you like wasting time, just don't show up here crying you cant find one.) ANYWAY, the module plugs in without tools, you need no dull-witted "mechanic" for this. The RCM is in the BIG electronics box next to the battery. THe lid comes off without using tools and the RCM will unplug without tools, but DO NOT pull on the wires, grasp the CONNECTORS to unplug them.

You will be on the road in no time, unless you let that "Mechanic" "fix" the car.

(If is sounds like I have no respect for "mechanics", that is not really the case. It's just that I am disgusted with them for not learning even basic electrical concepts, given they work in a very electronic vocation.)

RCM is in the yellow circle: