Trailer brake lights NOT working

Same problem with my 1998 KW and it was the trailer brake relay under the hood on the firewall.

 

Check truck fuses and relays. The relay will be labeled and just pull it out. Then pull one of the other relays out and plug it in that place. See if the problem goes away. If not then it is not the relay- except once in all the years I have been doing this. Once- there where two dead relays and I pulled the other one that was dead. Go figure. Now I carry a new relay on my truck and just use it to test.
If the relay is good and you don't have a blown fuse then the next is to borrow a known good light cord. Swap them and see. From there I would check the plug the cord plugs into on the truck. If the wiring into that plug is good then you are probably chasing a broken or corroded wire. Oh Joy.
And I would have to check a wiring diagram as I don't know off the top of my head if the trailer brake lights run through the turn signal switch or multi function switch as they call them now on that truck or not but if they do then that is also a possible place to look.
HTH

 

some trucks, like pre 2002 volvo have additional stop light relay for trailer, becouse truck stop lights are combined with turn signals, trailer brake light and turn signals are separated.

so I sugest truck problem

check trailer brake output on light connector first

 

Correct. By all means, take your test light or multimeter to verify that there is power going to and coming from the light cord. I should have said this first.

 

better use 55 wt bulb. sometimes bad connection lets measure voltage but not enought to light bulb