it sounds like you: run a wire from something that is hot when you want (i.e. key-on hot, or constantly hot - depends if you want your fan to run with the key out). i prefer to use a key-on hot only wire, so i cant forget to turn my fan off and drain the battery. find a circuit in the fuse box that meets your needs. dont worry about draw, as you are only powering a coil in the relay (probably 300 mA draw at most, or .3 amps).
run the wire from the fusebox to the switch's 'power' terminal. then run a wire from the switch's 'accessory' terminal to terminal (or wire) terminal 86.
basically the fan controller sends 12 volts to terminal 86 when it is time for the fan to come on. your switch will supply the 12 volts when it is not hot enough for the fan controller to kick the fan on.
did the instructions say anything about using a blocking diode (i.e. 1N4001) on the controller wiring (between the controller and terminal 86 on the relay)? i might add one, but im very cautious.
good luck, J.