Up for sale is a rood used Mercedes 85 Mercedes Turbo Diesel AC relay.  Although it's primary function is AC compressor control, it acts as a pass through for many other electrical items in the car.  In my personal car (an 85 300-td) - I didn't even have working AC, this relay did however cause a problem, because it was not allowing the kickdown signal to get to the transmission from the kickdown "button" under the gas pedal.  When this button is pressed, it would signal the relay and kick off the AC compressor, and kick down a gear in the transmission.  A bad one will allow neither...  You should keep a nice spare!