Hi Fellas, I have searched the site for a while but have not found an answer for my problem, hopefully you will be able to shed a little light on ...

Hi Fellas,
I have searched the site for a while but have not found an answer for my problem, hopefully you will be able to shed a little light on this situation....

I have owned this truck from new, I have not pulled a trailer for quite some time, but I have just hooked up to a trailer and did a light check...no LH signal or brake light on the trailer, everything else works.

1) I checked the light on the trailer, it is good. Signal works on truck.
2) I checked the factory plug on the truck, no power to the LH Signal.
3) I checked a secondary plug that was installed inside the box for a 5th wheel trailer, no power there either.
Note: secondary plug is wired from the front of the box, I can't see or feel if it goes to a plug or wired to the factory harness, the fuel tank is in the way, and the factory plug starts behind the rear bumper, plugged in with the factory harness.
4) I checked the fuse box in the cab by the Drivers door, all fuses are good.
5) I checked the fuse box under the hood marked LH Trailer signal, no power to either side of the fuse. (no trailer connected)
6) also checked the RH trailer fuse, no power to either side of fuse there. There is power to the RH signal at the plug. (no trailer connected)

What am I missing?
If it were a ground problem, there would be issues with all circuits of the plug, correct?
I'm not sure what to check next without taking it in to a service station.

Thanks in advance for your assistance.