Tony, two things to look at, first the bulb, at the RF turn / park light, Replace it with a new bulb, check the wires that go to the side marker light also, Is the bulb in the side marker good?
If neither of that works then the RF light has a bad ground, there is a ground wire from the socket, follow it and make sure it is attached to the body and a clean connection.
Maybe the ground wire has broken away from the socket.
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