If I recall this thread is about a 126 body car 300SDL. if thats the case the blower is easy to access under the dash on the passenger side. Looking at the power to it at the two pin blower connection can tell a lot. The 126 body is also notorious for having the ignition switch kill the blower only. A simple wiggle of the switch is all that is necessary to diagnose that one if the problem is occuring. Be sure that there isn't a hang-on fuse for the blower; the fuse box terminals weren't designed for the blowers load and many circuits were removed.
Bosch Master, ASE Master, L1
33 years MB technician