To test to find where the problem is for that. first you need something like this.
http://www.harborfreight.com/cpi/cta...emnumber=90899
set it to check resistance. unhook the dimmer switch and try each connection in all combinations. If you have contact it will be a short between those 2 wires, or bad headlights. So then unhook both headlight plugs and try one more time. if short is still present replace those 2 wires.If the short isn't present after unhooking the headlights but was before, then replace the headlights. If you don't have contact either way try the same test on the dimmer switch. If you have contact between more than 2 of the connections at a time the switch is bad. If all of that tests out as ok hook it back up. then hook a piece of wire to the neg terminal of battery and slide it in with the ground connection on the headlight. either side should do. If the problem is still there replace both headlights. If the ground wire makes it go away run a new ground wire to the headlights.