What can I do to fix this?
If I was you, I'd have driven back to body shop and raised hell. Unless somehow it's a coincidence that you're having electrical problems after they installed a kill switch. Seems to me like you paid money to have your truck screwed up.
It could be the headlight switch, could be the battery ground straps -- heck, have you checked
all the fuses?
The headlight switch will depend on year, but you're gonna want to take the plastic dash around the instrument cluster off, and the switch will be attached to the metal dash. At least on ye olde trucks I own, it unscrews and pops out the back, and you then unplug the big electrical connector from it. Installation is, as they say, the reverse of removal.
And of course, this being an electrical thing, you've disconnected the battery negative terminal(s) prior to dinking with the dash. Which gives you a good chance to examine the wiring for poor connections, corrosion, etc.
All of that would be in your Chilton's or Hayne's, the best fifteen bucks the budding mechanic can spend on their truck.
But I'd start at the most recent change to the truck.
-- A