Larry -
I understand. Different kind of bulb than I was assuming. You are right - the bent wire loop bulbs are not polarity-specific. However, one end of the socket brings +12VDC to the bulb. The other end of the socket goes to ground. You have to figure out which is which, and connect the red lead of the new LED to the +12VDC end of the socket. You might be able to see which socket wire is connected to ground. But if you can't tell by looking, the next best way to discover which socket wire is positive is with a test light, as you said. Plug the camper into the tow vehicle, and turn on the running lights. Pop the lens off the running light (you can leave the bulb in place), and connect one end of your test light to a known ground (frame, etc). Connect the other end of the test light to each end of the running light bulb, one at a time. The end which causes the test light to light up is the positive end.
Warning - you should do this for each running light as you install the new LED. There is no guarantee that each socket is physically wired the same as the others. In other words, if the left-hand side of the socket is positive in one fixture, there is no guarantee that the left hand side will be positive in the next fixture.
Perhaps a silly question, but why are you bothering to replace the running lights with LEDs? Lots of work, lots of expense ... The existing bulbs are plenty bright. Lower power drain doesn't mean anything, since the running lights are powered by the tow vehicle's alternator. Running lights almost never burn out, and if they do, the temporary loss of a running light isn't a big deal safety-wise. Just curious ...
Bill
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