I found it a wrecking yard. On my 2001 it plugs into a connector on the factory wiring harness at the left rear. You've got to remove the spare tire carrier and the plastic trim piece to reach the connector, you've also got to remove a rubber plug at the bottom of the fender space and fish the harness through. Fishing the harness through and reinstalling the rubber plug (which you have to cut) was the hardest part.

The bonus is that the wire to connect the brake controller to is behind the kick panel in the drivers side footwell. If my memory serves me right it's blue.

 

Thanks!

 

Hold on a second, my 89 XJ has a blue wire near the fuse box that is labeled e-brake controller or something on a pice of factory paper tape! Lol! Guess the color didn't change. It's even plugged into the fuse box and everything else but not the actual controller. I have the factory trailer plug that has to be spliced into the trailers electrical wires but the socket at the hitch is all corroded. I assume that needs to be replaced? And Do I need a specific e-brake box?

 

I bought an e-brake controller from e-trailer.com and mounted it under the dash. You've got to run a power wire from the engine compartment, find a ground, splice into the brake light signal cable and connect the blue wire to the controller.