Weird 12v Electrical Problem

[gmctoyman]

My SIL was installing a replacement clearance light on his toy hauler, it only had one wire (case grnd) and he couldn’t get it to work, so I got involved. As his TH was parked where we couldn’t easily get the PU connected I shorted the pigtail to power the marker lights & started checking. The light would not work properly installed. Knowing most LEDs were polarized I reversed the wiring and the light worked fine. I checked V and found hooking the leads on the meter properly gave me a negative V reading, hooking the leads reversed gave me the proper polarity, with a slight V difference. I checke V to chassis on both leads, the green gave me a proper 12v read, the white gave me .24v ? Checked continuity, both green and White showed 0 ohms to frame. Wired the new light in with reversed polarity and it working, but I’m still confused....
I figure probably a shorted ground someplace, but how can I find it?
I’m lost ??

[MrWizard]

i would check the connections at the TT pigtail
and at the truck connector for the TT

one of them might be reversed

[DrewE]

Were you checking continuity with the socket/wire still powered up? That will confuse an ohmmeter 100% of the time (and, in many cases, can damage it, at least if the applied voltage is sufficiently high).

If the other clearance and tail lights are connected to the same circuit and some or all are incandescent lights, there would be some continuity to ground through them. It wouldn't be precisely 0 ohms, but it would be a comparatively low resistance, probably less than 10 ohms.

If everything is working properly, I would not worry about things in this case.