With PLUG CONNECTORS all in GOOD Condition, I Would Be Sure Of OUTPUT From Both Sorces AT TRUCK OUTPUT PLUG If No Power there>> Isolated at TRUCK<< If POWER is Present Going INTO Trailer Try a TEMPORARY GROUND at each Bulb .
This Sounds like A Problem that Would ORIGINATE at teh > TRUCK /TRAILER CONNECTOR
Check the light mounts on the trailer's right side. It sounds like the ground is not connected on that side or the light bulb socket is broken or loose. A bad groundis usually caused by either a broken wire or rust under where the light is mounted. If there is rust, clean with a wire wheel then remount the light and test.
If the bulb socket is cracked or loose to the touch, replace the light mount but clean the mounting area as above.
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The trailer works fine on another truck so the problem is in the truck somewhere.
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