Why wont my running lights work on my truck?
Likewise, people ask, why are my running lights not working?
Common causes for this condition is a loose fitting bulb inside the socket. To test for this condition expose the tail light bulb in question. If the complete system flickers (all bulbs flicker) wiggle the tail light fuse, relay and associated wiring at the headlight/tail light switch.
Thereof, why are my running lights not working on my trailer?
A bad or missing trailer ground is a VERY COMMON reason that lights don't work on a trailer, or they only work sometimes. A pin on the truck socket and a pin on the trailer plug are dedicated for this ground connection, so test that connection FIRST when troubleshooting a problem.
A blown fuse usually causes both lights to go out. On newer cars, the tail lamps can be fused separately and/or each bulb or shared circuit can be fused. A fuse can be blown due to other problems, so you'll need to check more than just the fuse, but it's the right place to start.