It's probably not prewired, i haven't heard of a KJ that didn't have the console installed by the factory that had the wiring included. The install however is not difficult. Directions here:
http://earth.prohosting.com/jsalbre/evic/
Most of the directions apply to 05+ models, with the exception of the temperature sensor. It should already be in the grille and no additional wiring is necessary. I found the hardest part was cutting the headliner If you don't have a pigtail you should be able to use a slightly modified floppy drive molex connector to go into the evic. If you want a really clean install, you can get a connector for the odb port
here. You will also probably need the plastic retainer that attaches to the roof and holds the console in place. The part number for an 05 is 5072348AA and should cost around $25 dollars from the dealership.