On my truck listed, i have a western unimount, bought it in 99 i think.
When i plug in, everything works, but lights arent switching on the plow.

Where do i start? does a relay control this. I pulled open the lights , bulbs look in ok shape. what next?

 

use a circuit tester, and start following the harness back , and checking each plug along the line until you find power. The problem will probably reside close to there.