I have read anecdotal reports of similar climate control problems like you describe, caused by faulty ignition switch. Seems some previous owners may "overload" the switch by hanging a "Bunch" of keys on multiple rings, thus causing problems... Its a stretch... but has been reported. Try wiggling the keys in the switch.

I hate to imagine further...

__________________
RM Smith
1988 560SL

"Where is it again that we are going, and why are we in this handbasket"?