This is a long shot but, take a drop the positive lead of your DVOM in the reserve tank, ground the other and set you range to 20V. If you get a reading over .9 v, you really need to change your coolant, its acting like a rude battery. (seen this drive ford and toyota sensors crazy, for some reason alloy and acid in a glychol bath will do it)
Also make sure to pin out the harness, while I've not seen a SLK 111 harness failure it could happen.
Project Smoker, '87 603 powered wagon
Hauler, 96 CTD can you say torque?
Toy 73 Cougar xr7 convertible
Acme Automotive Inc.