If it's what I'm thinking of, it is a black square relay looking thing with a bunch of white writing on it, and it should say "Klima" on it pretty prominently. ("Klima" is German for climate, as in climate control system). On my '87 300E, there is a black plastic divider behind the battery that you can just pull out (it is not held in by anything), and the Klima, overvoltage protection, and another big relay are right behind it.