I would pull the instrument panel, and disconnect all the plug on connections and inspect them for corrosion. IF the car sat for a while, connections can corrode. I would also remove the ground bolts for the instrument panel directly behind the panel and wire brush and clean all the rings, the bolt, and the spot where they contact the frame.

I would also trace down, disconnect, wire brush and clean off all the multipin connectors and electrical contacts I could find. Include all the fusebox relays and the control module pins. If you have a badly corroded connection that is bleeding voltage to it's neighbor "contact pin" you could get some wierd symptoms..

If you don't have a enclosed area to work in, pick a nice day and begin in the front and work your way back disconnecting, brushing contacts and reconnecting them. I really like the mercedes weather proof connections to some of the modules, but it never hurts to brush off any oxidation even if they seem clean.