The headlight motor has no internal ground. Reversing the polarity of the motor feeds reverses the direction of the motor.
The DPDT (double pole, double throw)headlight relay reverses the polarity to raise or lower the headlights and the limit switches stop the travel.
I don't have a wiring diagram, but I think it's possible to see +12v at the motor but the limit switch interrupts the ground side, and the motor can not run.
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