I can't remember since it's been so long since I replaced my 280E climate control system - Mercedes' worst idea yet - with a manual system from a 240D. It may the relay for the CC in the box with the rest of the ice-cube relays on the left front wheel well.

Mercedes has a 2-disc set for the 123 car from their service manual library that connects online to their computer banks. I use it all the time, and the wiring diagrams are complete.