Could be the ground ? If your is like mine, the blower motor
mounts to a * fiberglass * box, and has a ground strap from motor
body to wiring, or direct ground to engine / body. With blowers turned
on, wiggle ground, see if motors start [ sputter than run smooth ]
Usually, power works, or it doesn't. Grounds can get sneaky
due to dirt build up, loose screws, excessive paint that flakes off
leaving connection loose and corroded. Beefing up grounds on * any *
car makes sense !