I can't answer your question, but the same thing happened to me tonight.

On my drive home, air from vents became luke warm, and finally, cold (same as outside temp).

I parked the car and started her back up and heat blew as it should. shut it back down.

Will see what develops tomorrow. Good luck to both of us.

Mike

__________________
'90 300SE 298k
-300K and it gets put into retirement.
'80 300D 255k Purchased new by family in 1980.

Had a:
1973 220 (gas)
1980 300SD
1992 400E