I had a similar problem in my '86 560 SL. I don't know if the cars are the same but here's what they did to fix mine. Behind the seat where a jumper seat would go on the vertical part there is a a capacitor can fairly large, with time this part can oxidize and rust breaking the electrical connection for the air bag. My mechanic replaced the part and it was solved. He also told me Mercedes had a bulletin and suggested that the canister be installed with the electrical connections facing downwards. Hope this helps.
Larry