I have an 01 dodge ram sport and every time I turn on my headlights both lights are really really really dim, but when I turn on my fog lights then my passenger light goes out and the driver side light gets bright as normal. When I turn on the brights, the passenger side light turns on but is slightly dimmer than the driver side. I also noticed the other day when I turn on the switch for parking/fog lights the driver side headlight will still turn on.

Now I do have aftermarket HID's running through a relay wiring harness. Is it possible that this harness is somehow back feeding through the factory wiring to the headlights? I thought about maybe I should just upgrade the headlight wiring with it's own relay and avoid the factory wiring.

 

Check the back side of headlight switch. Double check all grounds. And do a search in the FAQ/DIY section on headlight relays. There is a good thread with an excellent wiring diagram that should take care of your problem.