Headlights screwy

I hope you guys can help me out here this is a pretty good puzzle, challenging in the least. I have a 2000 GMC Sierra, and I have seperate lamps for high and low beams. My headlights have gone wacky. when my headlights turn on, both drivers side lamps (hi/low) and my passenger hi is come on together but are all very dim. Though my passenger low is working correctly. Attempting to turn on my Hi beams makes no difference in the drivers side, though the passenger lamps reverse roles, the hi is now on correctly, though the low has become a dim bulb. All my ground have been checked and are working well, all fuses are ok, and i swapped the relays in the underhood electrical box, as well as put in new bulbs. all of which have not made a bit of change. I thought perhaps a relay could be stuck half open/closed, so I applied a 12volt power to the drivers side low beam when the headlights are on, it shuts off the hi beams from being dim, and turning the hi's on works now without the lows being on dim, though both drivers side bulbs remain slightly dimmer than the passengers.   In my words, Wha-da-fa is going on here.... Help me please!

What have you done to address the in car switch, which activates the hi / low beams? Normally it is in the turning stalk. That would be my next step in the trouble shooting procedure, as you have stated all others *appear* to be fine. Regards EVIL Teken . . .

Thanks for responding, I have swapped the headlight switch in the dash with an equal switch from a like vehicle, no difference, i have not tried changing the multi-function switch due to the cost of getting one ($289 i was quoted) just to try for now.   I may try taking one out of the other vehicle i was swapping with, but didnt want to tear my brothers steering column apart if i didnt have too....   is that what you think the most probable solution would be? a new multi-switch?   Hope not, but let me know!

It could very well be the multi-switch which you have indicated. The same problem can also be caused by a power wire being allowed to shaffe, or be pinched, then touching some part of the chassis ground. But you have indicated that all aspects of the wiring is indeed sound. So that would eliminate that as a cause. I would however varify that all connection points are all firmly in place. Remove each one and use a pair of pliers and crimp each one closer together, to assure a firm contact point. I would also take a serious look inside of each one of those connectors which go into the relay pack to ensure that they also are corrosion free. There isnt much left in the circuit, other than to varify that all ground points are indeed clean, and free of any corrosion build-up, and firmly in place. Regards EVIL Teken . . .