Bert,
I must have done the same search you did...actually found a post with the same title as this one.
I didn't see any definitive answers, so here are some thoughts. If the heater motor works at all the different selected speeds it means the resistors are OK. The motor should pull more current when it runs at the slower speeds than it does at the higher speed (I think), and you're saying it only blows the fuse when run for awhile at the HI speed.
Does your fuse box indicate what size fuse should be in this circuit (10 amp), and are you using it?
Could there be a restriction in the flow of air passing the motor that might make it heat up when running on HI?
Do you hear any noise from the motor when running in HI that might indicate dry bushings or some mechanical issues with it.
If the fuse is sized properly, the motor would only draw excess current if it were crapping out. This would be my first guess. Your choices would be a intermittent dead short through a wire or bad switch, or a motor that was shorting or pulling excess current when it ran for awhile at max RPM.
I don't know how difficult it is to replace the motor, but aside from this, crawling around under the dash, wiggling wires in a effort to get it to blow a fuse would be worth a try.
If the resistors are contiguous, they are probably fine. Keep us posted.
Stuart
