Glad you found that the problem wasn't with the lighting.
For those who are having the same problem not because of a loose connections, or similar.
I found some vehicles dim tow lighting becomes a problem due to wiring voltage drop, bulb type and how the reflectors are desig
Installing a Roadmater tow vehicle lighting harness to my Jeep using Roadmaster diodes. The signal, brake and running lights worked great except they were way too dim.
After looking around I found Draw-Tite Modulite. The instructions show how to install into a towing vehicle which apparently it was originally design for.
Wiring:
To Trailer: GND, Tail, Left Turn/Stop, Right Turn/Stop
To Towing vehicle: GND, Tail, Left Turn, Stop, Right Turn, Battery
Instead of installing into the towing vehicle I installed the module on the Jeep reversing the wire connection. The Jeep has fused 12 volt from the battery GND wire to the module. To Trailer: (goes to towing vehicle) To Towing Vehicle (goes to tow vehicle lights)
The module is installed inline with the Jeep's tow lights wiring harness, inside the engine compartment. I also installed some waterproof spade connectors so the module can be removed when not in use.
The trailer/tow lights are now nice and bright while towing.
The one I have is Draw-Tite model number 18187. (apparently has been discontinued)
Draw Tite 18187 ModuLite-Lite Trailer Light Power Module For Circuits Up To 4 Amps - FastTrackAutoParts.com
Curt has a similar module that appears would also work, at least for my application. If your tow vehicle lighting is more complicated, may require another type of module to amplify the trailer lights current.
https://www.etrailer.com/Custom-Fit-...urt/56146.html