On the firewall on the right side at the top, is where the mono-valve is located. It should be a little silver cylinder about 1/2" tall that is held on by a couple of screws. Suggest you order a new one, unscrew the old one and slip it in. Be careful that you don't break the fine copper wire that is part of the copper wrapping. You shouldn't have to pull that part out (what the valve sits down in) to replace the valve, but be careful if you do.

I have a dual climate control system on mine w/two valves. My mechanic broke the wire when one was rusted and he had a hard time getting it out. That's the only reason I mention it.

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Don
'85 500SL (Euro) - 186,000 w/a complete restoration and engine rebuild at 154,000
'95 C280 - 174,000