If you are testing the left brake/turn signal lights on the trailer right at the socket of the assembly itself and getting power, but the lights themselves don't work when the bulbs are in place then my first thought is a weak ground connection on the assembly.
Since you would be grounding the circuit tester as well, this basically could be strengthening the ground enough for that circuit so that you get power when testing but not when the bulbs is back in place. Trailer lights will either... view full answer...
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