Turn signal and 4 ways working on my trailer but not my running lights or break lights

Get a test light and check the end of your pig tail that goes to the trailer. If everything is good there then the problem is on the trailer. If you don’t have any juice at the end of the pigtail then check the truck side. If everything is good on the truck side then the problem is the pig tail.

Other things are spread the tongs out inside the trailer box for a better connection and get something to wedge under the plug so it fits snug when you plug into the trailer.

 

Sometimes the receptacle on the trailer will separate inside. Remove the two bolts, pull it out and make sure the harness is tight.

 

No self respecting truck driver should ever be without one of these and a few other simple tools

 

As mentioned..test light the plug. If nothing there. Test light the truck plug connection.

You didn't mention what truck but you might have fuses for the trailer lights also. Test light the fuses. If one side lights but the other don't. The fuse is blown.

 

If pig tail is ok...
Some wiring harness's use another plug to plug into the back of trailer plug ( Transcraft for sure )
Their is a little white rubber plug that holds these together. You may have to clean the back of plug to see the little plug.
new plug it around $100 so be careful not to force it if this is what you got. Don't take much to cost you a $100 bucks.
The GROUND may just need to be cleaned ....

 

Caig DeoxIT contact cleaner. Almost every time I’ve had trailer light issues, spraying this on the trailer contacts and into the pigtail has fixed it.

 

First try and wiggle the plug where it plugs into the trailer. Otherwise get your self a test light or one of these.

 

The first thing I ever did was take a pocket knife and GENTLY split the trailer receptacle thingys on the trailer. It doesn't have to be much, and too much will break them off. Just split them to make a tighter connection. Also, a tube of electrical grease (yeah, there's such a thing), will help as well.