I had my fog lights replaced after a knucklehead decided to test the crushing strength of my 2500. No damage under the hood other than small indentation along the tranny cooler grill, and bumper had to be replaced with a larger steel brush guard and idiot deflector.

The head light did not dim until I had the new fog lights put in. I used a performance shop for the install and they couldn't wire to my pull switch on the headlight switch. They put in a toggle. No problem.

I checked all the grounds but can't find one that has an issue. Any thoughts?

 

I changed my headlight bulbs the other day and one side would only work very dim. I pulled the bulb out realigned the pins, cleaned the female pins, and reinstalled. Finally it worked!!!
Maybe it's a bad connection, check the bulb pins for alignment.

I hope this helps