If the fuses are good and there is no power on only one connector pin..you will be tracing for a short or corrosion for lost connection.
Have fun with that.....lots of time and most are not successfull.
If you are not successful, I would run a new wire and relay off your rear tail lights to get a switched running light and feed it onto the truck/trailer wiring harness. They make fused/relayed trailer wiring harness that will plug right into the tail light wiring so you won't have to splice into anything. Just tap into a power source for the relay activation.
Goodluck...you could always seek profession help from someone like me who does this for a living. It might be the most painless fix. There is a possibility that you missed a fuse or the trailer harness connectors just has some corrosion on it.