My first guess is a bad blower motor. I have never worked on one so I don't know where to get at it.
I did do one on a 123, under the dash on the passenger side. It was rather easy. With a 124 body I don't know.
I will do a search also .... I will post the link if I find it.
'03 E320 Wagon-Sold
'95 E320 Wagon-Went to Ex
'93 190E 2.6-Wrecked
'91 300E-Went to Ex
'65 911 Coupe (#302580)