On my trailer plug in only the running lights work! (put a 12v tester on , not a trailer) But every thing works on the truck. Any ideas?............problem solved......................had two blown fuses for the trialer light plug, in the fuse box under the hood ( on the driver side inner fender)

 

check your plug on the trailor maybe the wireing inside is loos there should be some screws that hold in the wires loosen them check he wire to se if its broke inside there
or tighten up the screws if there loose
9 times out of ten at were i work its the plug having loose wires
also if that isnt the case maybe there is a short in the wireing harnes on the trailor
hope this gives you a place to start

 

I'd say crawl under the tailor and look for grounds, and make sure that the bulbs in the trailer are good.

Try plugging the trailer into another truck and see if it works there.

 

Did the lights ever work or is this the first time you tried them? Are the contacts in the plug clean? Does your tester have a good ground? Does your tester work on other vehicles?