I have the same issue with my 1988 300E. I also have a hacked up AC wiring harness though. But the car is at the independent shop now and I should find out what gets replaced to make this incredibly annoying relay clicking go away. I have read posts here that point to a blower regulator relay being a problem. On the E that part sells for 250-300 bucks. I'm betting on the CCU itself though.