I hate to be the bearer of bad news but your car has what is known as an ASR 1 system. This was before they were thoughtful enough to have built in diagnostics. They are a nightmare to diagnose, and I try to hide in the shop when one comes in. You have to do a breakout on the abs and asr modules looking for problems hoping to find one, with no guarantees. So what I'm getting at is that a mercedes tech with all of the special breakout boxes and cables and a lot of know-how has about a 50% chance of diagnosing a complex ASR 1 problem. Take that for what you will. At the very least it may be worth it to pull the connectors from the module, the actuator, and potentiometer to make sure corrosion hasn't crossed the circuits inadvertantly powering up the actuator.