Get a test light and manually test every prong on your truck's connection. I have a "map" somewhere of different typical layouts, but quite probably it's in my glove box or in the trailer (which is now in a snow bank). Clamp on the ground, touch each prong appropriately as someone else is initiates the lights for the brake, signals, parking, hazards. Any problems at all, do the same thing, but use a part of the truck's frame, well scratched, for the ground. The plug on my truck was factory installed, but all of a sudden I started having trouble. I pulled the plug and found that they had only used 1/2 of the wire for the ground and didn't leave enough slack on top of that and I basically had a ground only when the moon was purple. :)
If you SAW that the lights worked on the other person's truck and you determine that your truck is fine, the next is to check the ground on the trailer and then ensure that the trailer connector is properly wired. Sometimes people do weird things to connectors - messing up the layout.