I would try to bypass the switches (not fuses though) and see how the fan runs without the controls ahead of it. Not too hard to access. If the brushes are worn out, the problem should persist. If the problem is upstream, then the blower should run fine.

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John Schroader
bio burnin' 83 300D, '83 300 SD, '79 240D
"I've never met a man who was good at making excuses who was good at anything else" Ben Franklin
"You cannot permanently help a man by doing for him what he could and should do for himself" Abraham Lincoln