That's the big problem, mechtech2, If I could take the unit apart, I'd go over it with a fine tooth comb. My first attempt at taking the unit apart was unsuccessful and resulted in many cracks in the plastic unit. My first posting for help on a Mercedes forum didn't get me anywhere.
I have the wiring diagrams, but it doesn't help if you can't get to the wires. Some tasks on this car are rather straightforward. Others, like this, are a nightmare.
Dishdude is right, these cars are needlessly overengineered. This car has had a serious problem like this come up eery 3K.