I FEEL YOUR PAIN! I too am working on getting my XJ to run cooler.
I started off with a coolant flush, which after all of the previous owners stop leak was washed out, resulted in a new water pump. It did drop my operating temp down to around 190 on flat roads, 200 when climbing a hill.
I am about 60% through wiring in my own fan override like the one you saw (I saw it too). I went and bought an automotive relay, ran a piece of multiconductor cable into the cab. The relay is out by the relay/fuse box, where I spliced into the leads going to the fan itself. I tried using the relay that was there, but had trouble determining the control wiring and decided to add a relay rather than risk melting down the computer. All I have left is wiring the switch inside.
I do wonder if there is some retrofit switch out there that can replace the existing fan switch and turn the fan on at 200 rather than 230...then all this would be unneeded...
Here is an article about doing the electric fan/belt driven fan swap:
http://www.off-road.com/jeep/cheroke.../cooling1.html
I am planning this one and also (when my radiator pukes) I am going to replace with a three core radiator I saw here:
http://www.extremeradiator.com/Produ...spx?sku=1193-3
God Speed!