Heater motor fuse popping

[cppursell]

Hi all! My '67 C10 is still on the road, and hauls my '62 Airstream Overlander (I live in it full time). As you know, it's often a joy but sometimes frustrating to keep everything working, and this time I'm stumped... may I get your insight?

Heater fan motor keeps popping it's 10a fuse. ...I've already replaced the motor, switch, speed resistor. I actually bypassed the fuse panel and am using an inline 10a ATO fuse. I inspected the original heater harness and it's in good shape. The connection between the fuse and harness connector is good, multi-meter doesn't seem to show resistance. Motor is grounded well.

Symptoms: When I replace a fuse, the motor runs at all speeds when the engine is stopped. When I start the truck, the fuse pops when fan is at high speed, sometimes lower.

I'm defeated, do you have any ideas? Only thing I can think of is a failing voltage regulator, since I notice, momentarily on occasion, the power fluctuates and the engine sputters at idle?

Thanks for your help!