This is on my wife's 01 Sierra. I was at the track this past Firday night and took off the passenger side headlight. After putting the passenger light back on, the lightbulbs light up very dim. This is for both high beam and low. This is the weird part. When the passenger lights are connected and and in low beam, the high beam bulb are lit as well but very dim on both sides(passenger and driver's side). This does not happens to the driver's side when the passenger's side is disconnected. BTW, the lightbulbs are perfectly fine as I have tried them on my truck.

 

Sounds like a ground issue...I'd check and make sure that none of the wires got accidentially partially broken or backed out of their connectors, etc.

 

agree, or some old/damaged wire

 

you sure you didnt plug them in backwards

 

After getting pissed by not finding anything wrong with the wiiring I decided to check the fuses... Bingo.. Problem stolved.