I extended the factory wiring for my previous LED 3rd brake light and added a plug to the end of the wires.

Here's a close up view.

The project consists of an open hole center cap, 2 1/2" round grommet and a 2" round LED light. I re-used the 2" light I had with the Moab wheels. Somesay I will upgrade it to a 2.5".

The 2" LED fits in an inner recess of the larger grommet.
A 2.5" LED would fill the grommet.

The grommet is meant to fit in to a hole on a trailer, etc. Used here, the lip of the grommet fits in the center cap and outer rim fits over the center cap giving a nice snug fit.