On my 1971 Tradewind, there is such a small box on either side, and they are light monitors. I can see these lights work in unison with the turn signals and brake lights in the rear view mirror.
If this is what you are talking about then you should have them on both sides.
They work with fiber-optics, I believe.
Uwe is correct. They are fiber optic light monitors that come on when they senses your stop/turn lights. Not all trailers had them and many were only on the street side. If you take your tail lens off you should see the light sensor protruding though the lamp assembly. Sometimes they vibrate out of the socket so that might be all that’s wrong with them.
Randy
I have them on my 1974 Overlander (27'). They are fiber optics. Mine light up in a "white" color when ever their respective lights are on (turning or braking). Kind of a neat way to verify that the lights are working by just looking into your tow vehicle rear veiw mirror!
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