Hey when you solve your high beam problems, let me know what the solution was. I've got a Dodge 2000 and the last time I went to hook the plow up, my plow headlights didn't work (low or highbeam). I replaced all the relays ($28!) and cleaned all the electrical connections but still no lights. Finally, with another snow bearing down on us, and another plow with much more serious problems that needed attending to, I just hot wired the lights to the battery and put a toggle switch under the hood to get me by. Worked fine - but I'll need to fix it right. Maybe your thoughts would send me in the right direction. Otherwise, I'm headed to the garage with a multimeter and a wiring schematic.
By the way, if I hadn't found a wiring diagram in my owner's manual, that schematic that was submitted would have been an absolute godsend! The people on this site are the best - always willing to help out a brother landscaper in need!