My XJ has been pulled behind our motor home for over 60,000 miles.
Not wanting to cut into the Factory wiring, I ran a separate tail light system, used when towing.
There is room in the twilight housings for an added bulb and socket
https://www.flickr.com/photos/lwade/3491989243/in/album-72157603323570942/
I used this kit.
https://www.rvupgradestore.com/Bulb-Socket-Tail-Light-Wiring-Kit-69-p/bx8869.htm
Diodes drop the voltage, providing less lumens where you need them.
Bypassing the undersized Factory taillight wiring and using dedicated wire, without diodes, provides the maximum voltage to the bulbs.
Also added a LED center brake light For better visibility .
https://www.flickr.com/photos/lwade/4117021964/in/album-72157603323570942/
Yeah, that's probably how I would have done it in the first place.
The lighting kit and harness was originally installed by a professional welding & RV shop in another 4x4 that was eventually wrecked off-road. I tore out the wiring harness and installed it in the XJ, then found a tow bar plate for the XJ so to be able to use the same type of tow bar.
Tow bar equipment can become expensive, when purchased new, sometimes run well over a couple of grand if you have them installed along with a brake controller. I've used the inertia portable type and they do work. You then have a set of 8 brakes vs 4 brakes. Stopping is much quicker & shorter distance. For frequent hauls in traffic and cross-country hauls I would recommend a tow brake.
I've purchased new tow bars but also have seen used one's still in good condition at RV shops for much less. Once in a while tow bars show up at garage sales, on craigslist, etc. still in good shape. There usually are repair kits and spare parts available for tow bars. If you're good at welding you can fabricate your own tow bar plate, etc.
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What I did to fix the low light output, was install a lighting voltage amplifier kit, such as a brite-lite kit. The kits are designed to be installed in the vehicle that's doing the towing, but you can rewire them so they work in a vehicle being towed.
I have the amp kit wired so when requiring tow lights I plug the lighting amp. into to the XJ's tow lights wiring harness from inside the engine compartment.
I normally keep it unplugged and stored inside when not in use.
