not fishing but boat related.

[branhamautomotive]

Many places sell a simple led tester that will quickly give you an idea if your vehicle is putting power to the plug, a test light or volt meter will do the same. I like a volt meter cause it can show voltage drop not just on or off. But 1st is to verify power coming from tow vehicle, if so check for power at bulbs and work backwards till you find it. Could be as stated bad bulb, loose, bulbs, bad connection, bad plug, broken wire. I have even seen some trailer lights have a separate ground set for stop/turn and parks lights, so 4 wires at the light not 3

[Lazy Ike]

Reel Deal,, What kind of truck are you pulling with... When pulling trailers in my old 2003 Silverado the running lights would never work if the truck light switch was in the auto position. I would always have to flip the light switch to the ON position. Just something you might try before you go chasing wires etc.... I bought new Silverado last year and have not had a trailer hooked to this truck to see how the lights work on it...