Sometimes the problem is obvious...

... once you get to it.

The heater blower in the car went inop a while back.  Normal checks were inconclusive.    Blowing the fuse must have been a recurring problem because the car ships with a resettable fuse (circuit breaker?).  But that hadn't popped.

I kept thinking it was a bad starter interlock relay or bad connection.  But the tests all showed good connections.  Finally I pulled the motor out and the problem became obvious. While installed in the car, this connection is only visible by standing on ones head and using an inspection mirror.  The connection to the motor developed a bit of a "high" resistance connection which gets hot.  And once the heat starts, the metal parts corrode and the resistance goes up.   This connection eventually became hot enough to melt the nylon connector and plastic housing together.  Oh and the insulation on that wire is supposed to be blue.

The good news is the motor still works.  All I need to do is cut out the cooked wire section and splice in a new wire & connector.  Then clean up the corrosion on the connector and reassemble the parts.  From here it's a twenty five cent fix.

Physics content:
Power is an I²R equation.  When I (current) is high, the almost any resistance is too much.  The current was probably in the 20-25 amp range.  Square that and multiply by 1 ohm at this connection gives 400-525 Watts.  That's a lot of heat!  Even that much current with a .1 ohm drop is still a lot of heat (40-52W) in such a small place.