Mel, that’s a universal trailer light I got at Auto Zone or somewhere like that for about $6.
I used a piece of slotted angle iron bolted to the back of the license plate holder to make a pad to bolt it to, then wired it to the driver side brake light. Twisted both of the new light’s wires together so all the filaments in it burn when the brake lights are activated. Also clipped a ground wire to the mounting post for good measure.