Originally Posted by Hittenstiehl
For those of you in the know can you tell me what this little red light might be for. It is located in the rear equipment area on the street side upper left corner.
This is the "polarity" light. It is a safety device of the era, but incompatible with modern GFCI safety devices. It was used to tell you if the receptacle you were plugged into had the load & the neutral reversed. This can be quite common in campgrounds that were wired by non electricians or a handyman who didn't understand. The theory was that if the load & neutral were wired backwards in the receptacle & the ground was somehow connected to the neutral in the trailer, the skin of the trailer would become live, causing an unsafe condition. The light would go on, alerting you to the fact that a problem was present. The way it works is that one side of the filament is connected to the ground & the other side to neutral. If the receptacle was wired correctly, the light would stay off, if not, the light would go on. Modern GFCI receptacles operate differently, & will "pop" if a vintage trailer equipped with a polarity light is plugged into one. This does not necessarily mean that your trailer is wired incorrectly, it just means that you need to remove the polarity light & try again. The GFCI operates around the principal of differentials between the three posts, i.e. ground, load & neutral. As soon as it sees a condition that doesn't suit its parameters, it'll pop.
Hopefully this helps out.