yeah on a plow setup you are supposed to have the 6 pin 3 way toggle switch with only 2 on truck or 2 on plow on at a time, i would be willing to say that somewhere in the line you cooked a main feed wire trying to power 4 highbeams off of a 2 light circuit... if you cant get the 6 pin 3 way switch you can always get a 3 pin 3 way switch and run a relay to power the plow lights off of 1 setting on the 3 way switch and the other position on the 3 way switch to the factory headlights, of course with the 3 pin 3 way you will have to wire the toggle switch after your high/low beam switch there... hope you good luck in finding the current issue, but easiest thing i can think is to get a cheap test light if you dont have one and start testing for power from the mfs back towards the battery/fuse block