Def not the way to set up a fan. He should at a minimum have some sort of toggle on/off switch. To the OP, you should really go and check the gauge of the wiring on the fan amd see if you have any sort of relay/fuse inline before the battery. Stock mustang wiring is notorious for being a fire hazard as it is, and adding any further load improperly is begging for trouble.
See if you can post some pics of the setup.