One thing I am unclear on if someone could help me out. Most fans say they need to be setup on a relay circuit. What is the point of the relay and will I need to do this for my trans cooler fan?
You need a relay anytime the amperage of a device is too high to run through a switch. A relay functions as a secondary switch that can handle much more amperage (current flow). When you turn on a switch, it sends 12v to the relay which closes a circuit in the relay and connects the device directly to a 12v source (with bigger wires). Here's a pic of a basic relay
The fan I installed draws 5 amps. The switch I'm using is capable of handling 20amps. The factory fuse is probably 15amps so that's the limiting factor. Personally, I wouldn't run more than what I'm currently running through the factory wiring... its pretty skinny.
To give you an idea of how much power the fan is using... we can use the formula amps x volts = watts. So 5x12 = 60 watts.
This thread has become the how to install a trans cooler gauge and fan thread, haha.