Like I said, it ain't no big deal. You must remember, you are reading volts, not amps or current. If you have your meter's black lead to the negative side of the battery or frame then the readings will be positive (or should be) on the meter. I would start with the feed side of the headlight switch. Is it 12 volts? Output side of the switch, both headlights and markers...12 volts? Dimmer switch input...12 volts? Output of dimmer switch both hi and low...12 volts? Move on the the light sockets. What do they read? If as you stated, you are ging to use a lead for your meter to ground and you see 12 volts all the way along then I think you have a bad ground in your headlight housings somewhere. It's a pain but I run all grounds from the lights to the frame. BTW, you do have a strap between the cab and frame and frame to battery, right? Good luck.
