It is most likely the blower motor regulator. You can find a regulator without the motor to save money. Very very common problem with these cars, also when you replace make sure to change the a/c cabin filter as this can be a culprit of burning out the regulator if it has not been changed and is very dirty.

Somehow our w140 got 2 regulators and never had the filter changed. Great MB service eh?

__________________
2001 G500 (45k and counting)
1991 350SDL (211 and counting)
1998 C43 (50k) Silver/Black (sold)
1995 E500 (40k and counting) Pearl Black/Anthracite (sold)
1991 300CE (50k and counting) Silver/Black Cloth (sold)
1983 300D - 79k Classic White/Blue Leather
2002 E55 Obsidian Black/Black
1996 S600 Coupe - 75k Black/Black (traded)
1984 300D -145k (sold)
Silver Blue Metallic/ Blue Leather
1985 300D - 87k (sold)
Manganese Brown/ Palomino Leather
2005 E55, 50k(sold)
Flint Gray/Black