If they are mounted by magnet and plug into a cigarette lighter type outlet, it is likely that the ground is carried in the wire rather than chassis. I would ask OP what happens if he wiggles the plug around? Maybe he already figured it out, since he hasn't been back since bumping the post yesterday AM.
Lights are on a custom harness, that isn't the issue. Just had them plugged into my truck and everything works fine.Ya, but the wiring on 12volt cig outlet may not be tight, or plug loose. I just know leds dont play nice with loose grounds. Could also be Chinese special lights. I had a set of ebay chinese special back up lights for my work truck. Didnt even make it thru 1 winter. I switched out to Cree lights 5 years ago, still going bright.
The cig outlet is dead for some reason.