I've done the same thing - bought used sport lights and installed them with my own relays direct of the batteries. I have a Curtis plow and everything worked well until I mounted the plow this year. When I switch from vehicle to plow lights, the high beam vehicle lights go on instead and the plow lights glow very dim. Figuring it must be a short but it's kind of confusing that low and high go on. I've tested the switch continuity and it is fine. With the plow off, everything works normally. Anyone have a clue on where I can start?Sharp truck! Yep, someone converted it to use the premium 4 bulb headlight system. I did the same thing to mine except I wired up my own relays.
Glad I could help!