I found a regulator in a salvage yard for my 1997 E300D for $75. They are in demand and tough to find. The dealer could not sell just a regulator but instead offered a new fan / regulator (expensive).

The original design does not allow enough surface area to adequately conduct heat away from the regulator and therefore the regulator malfunctions.

If you're lucky enough to find a regulator I suggest you do something similiar to what I did. Buy some conductive electronic paste at Radio Shack. Get some aluminum foil to increase heat transfer surface area and act as a heat-release-fin. Sandwich Aluminum fin between regulator and blower body. Smear heat conductive paste on all contact surfaces.

I did this two years ago and my system has not failed since.

Good luck.

__________________
Neil
1997 E300D - naturally aspirated
280,000 miles (as of November 2015)