the reciever has an open 3rd channel slot. you will need a plug end (the kind that are on servos) and take the black/red wire on the fan, cut it so it doesnt go to the esc anymore and wire it to the plug end and plug into the 3rd channel.
its not really that hard. you could probly find a "how to" with pics if you do a web search.

see, the stock ESC's fan is meant to run on 6v. thats fine when powering the esc with 7.2v nimh or 2s lipo. but when you up the voltage the esc give the fan too much voltage and it will burn out.
by powering the fan off the reciever, it will get a constant safe voltage no matter what battery you put in the truck.
you could also just buy a fan off ebay that comes pre-wired to plug into a reciever. then you just gotta figure out how to mount it on the ESC.