Had the same problem; blowing the fuse in the little coffin on my 89 300CE. After replacing some parts and not solving the problem I learned that the problem was in the push-button switch on the dash. As long as I use only the low and auto buttons the fuse doesn't blow. On the 'auto' setting blower still runs at max when needed. I took the dash panel apart but could not find any problems, so now I live with it. Haven't found a replacement panel in a boneyard and won't pay for a new one. Big $.
So buy a couple of fuses and experiment with the buttons. Then don't let your wife drive your car on a hot day unless you tape over the 'max' button.