1990 190E 2.6 defroster issue
 

When I put on my defroster, it will blow for about 1/4 second and die. The blower will not work at all after this happens. After a few minutes, it works fine except for the defroster. What is the issue here?

A couple of other questions. What should my rpm typically be at 70mph? I think it is too high; about 3500-3800. As always, any help is greatly appreciated.

john