Left front headlights dont work, they are on very dim but not like the right side. switched relays made no change. bulbs are new. 2003 GMC 2500 HD 4x4 with 6.0 vortec V8

The side that is dim has resistance in the electrical circuit , use voltage drop testing to find the problem . Watch this video on voltage drop testing .

https://www.youtube.com/watch?v=YaYtCVBodpw&t=1s

Look like you got a ground problem

Check voltage to lamp assembly. Could be a relay, a bad wire, or a bad connector. Since one headlamp works correctly, the switch assembly probably works. Without a wiring diagram. I can't tell whether the headlights get power directly from the switch or not... Time to buy a manual.

you may just need to change a fuse or two

I am sure that a 12 Volt bulb means it needs 12 Volts. Since you changed the bulb to the other side, why not check the voltage in the working side and see what it reads?

Sometimes the turnstalk can interfere with the lighting. Try pushing the Hazard switch several times and see if the headlights straighten out. I read that too much goes through the turnstalk and that using the Hazards can derail a contact inside the stalk.

If this does not help, you may need a headlight relay in the powerbox. One contact may be dirty or burned down for the wire it feeds. Unless you have a single wire feeding both low beams to the headlights, I would bet the left and right side come out directly through the relay.

Either both bulbs are bad(I've seen it before) or your
dimmer/headlight switch is bad. If you can without pulling the bulb, pull plug from the light. If it's a 4 lamp system pull the low beam. Do this on the left side as the right feeds from the left. With a 12v test light, check for power. (with lo beam on) if you have
power, change the lamp, if you don't it's most likly
the dimmer switch at the steering collum.
Hope this helps Good luck

I would suggest that the head light relay is not working ,or doesn`t have one If you check the voltage at theback of that headlight ,you will find it is well down fit a relay problem solved

go here. we can help you. Also, did you check the headlamp relay in the box under the hood? One relay is in box in front of battery, other is in narrow box along passenger side fender under the hood. swap left & rt headlamp relays to see if it changes.

http://groups.yahoo.com/group/villagerquest/

I would check the fuses under the hood for the left head light to see if it is blown.If not try switching the right headlight relay to the left one and see if the problem switches to the othere side. If it does switch to the other side get a new relay and put it in the side that isn't working.

You have to remove the air intake filter and then kind of reach around up in there to change it. It can be done, but is a lot of work. On the duramax, it is even harder to do the driver side because of the dual batteries. You have to remove the battery, then the battery seat plate, and undo the winshield washer fluid tank and lean it forward. Then you can just sort of reach in and change both headlights. A very big pain to change a couple of headlights, but if you want HID's.... for the average person, changing a headlight now means a trip to that garage.