We regularly have a similar problem on our 1984 190E Stage Rally racer - yes we still have the factory climate control unit in a race car, although we have removed the air conditioning compressor, condenser, evaporator and other associated bits for weight savings.

There is a connector on the back of the climate control unit in the dashboard that works itself loose and pops off, rendering the system inoperative - no fan, no heat, nothing. The air coming from the center vents is just outside air being force-fed by the movement of the car.

One of these days, I'm going to develop a method to positively lock this connector in place, but for the time being I just reach behind the unit and reconnect it; it helps to know what you're looking for when you're doing it by feel, which I unfortunately am familiar enough to do.

Your trouble might be something different, but I recommend that you check the simple stuff first.

Best Regards,
Rob

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Robert Fini

'12 ML350 BlueTec, 111k
'06 E320CDI (x2)
'05 T1N Sprinter 2500/Pleasure-Way Plateau TS, 72k
'97 C36AMG, 325k (son's)
'94 E320T, 249k
'93 190E Sportline LE, 168k (daughter's)
'84 190E-2.3/5spd (Stage Rally Racer)
'66 230 W110 Sedan (Barn Find, Vintage Racer build in progress)