Test it from socket to trailer plug, with all the bulbs out and the board not plugged into the car.
Use the lowest ohms setting (or Continuity if it's got it)on the multimeter. That way you'll at least know if the wiring is intact.
If you're not sure the bulbs are OK, test them the same way. You'll get no reading if they are blown. Compare the results of the 2 stop/tail bulbs. Sometimes (rarely) they internally short between filaments which can make odd things happen.
Then,test the earth side of the bulb sockets to a negative on the car (Towbar should be earthed, with the board plugged in and the car turned off.
there's no point in testing it on 12v DC-the light bulbs are a pretty good indicator that it's getting power
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