So, you have brake lights and turn signals, but none of the running lights work (neither the tail lights on the low-power filament, nor the marker lights)?
Since you have a vintage trailer, are you sure that you have the umbilical connector wired correctly for modern tow vehicles? Did these lights work before and stop, or is this the first time to set up this trailer with this tow vehicle?
If you're familiar with the pinout pattern and a multimeter, you can check for voltage on the truck's output pins. If you're not, or just don't want to fiddle with it, you can get a plug-in tester like this one to check the truck's brake circuitry.
Il Carriaggio — 1975 Argosy 24 | Il Progetto — 1976 Argosy 28 Center Bath | WBCCI# 15566
He has all of the virtues I dislike and none of the vices I admire. — Sir Winston Churchill