Check the bulb and insure it is not 180 degrees from being installed correctly. Old sockets are sometimes worn or out of shape and the bulb may sort of fit either way. You can look at the pins on the bulb to see which one is the "High or Low" entering the socket tracks.
Insure the bulb fully twists into place in the socket once it is fully depressed in. Sometimes the two contacts in the bottom of the socket are kind of sticking up too high and the bulb cannot twist. Sometimes you can pull on the wires on the backside of the socket to pull the contacts inward while you install the bulb.
If it were working recently (and the other side works) we might rule out a wiring problem. Has the wiring been chopped on/ monkeyed with to install a trailer light pigtail? If any of the above is not working we might do some testing. Do you have a twelve volt test light or a voltmeter?
The Medic
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Friday, April 13th, 2018 AT 9:11 PM