The easiest test is with a Volt/Ohm meter. Set the meter on Ohms, touch one probe to the side of the bulb, or if a sealed light, to the white connection. And the other probe to one of the balls of solder on the bottom, there are two so test each one, or to the brown wire connection on the sealed bulb. The meter should read 0 or close to it if the bulb is good.
If the bulb/light tests good, go to step 2
Connect the trailer plug to the tow vehicle. Remove the light plug at the non working light. Using a test lamp or a Volt/Ohm Meter touch the ground wire of the meter or the test light clip to the white wire of the plug and the plus wire of the meter or the pointed end of the test light to the Brown wire of the plug. The Light should light or the Meter will read 12 volts. If it does, then you have a bad bulb/light if it’s a sealed one. If not, go to the TRAILER side of the plug, you’ll have to take it apart, and do the same. If you have power there, then the problem is in the wire, probably the brown wire as you have power on the turn and stop light, and you have power to the other side tail light. In most cases there are 2 Brown wires, 1 down each side. Look in the wire loom to see if there are 2 and one may have come unhooked. You may only have one tho. Also check to see if the clearance light works on the bad side. If it does, then it’s probably the light plug itself. Trailer wiring isn’t rocket science, but sometimes it does seem that way. Always start testing at the affected light and work back towards the power source. Good luck, hope this helps.
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