The W201 chassis (190's) has a unique A/C system!
Almost always when you hear a clicking from the right side of dash you have a problem with the "feed-back potientiometer" that sits on the blend air door.
When the potientiometer gets a bad spot it sends back BAD info to the pushbutton & the pushbutton cycles the switch-over valve on & off very quickly!

HAVE replaced at least 100 for that problem, dash must come out for replacement, however you can remove glovebox & plug in a new one to see if that is the problem. MB part number 201-821-02-60. Good luck

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