Did some research, got well into this job today.
Three pin switch, load, +ve, -ve
Fan relay is above brake side footwell, closest to tank of the two.
To install a manual over ride switch, while still maintaining the sleds ability to start the fan automatically if temps creep high and you weren't using the sled to groom, you tie the switch between the Brown/Green wire before the relay, and the Blue wire after the relay, i.e. the blue wire on the two wire connector that feeds the rad cooling fan.
The Brown/Green wire goes to the +ve terminal, the Blue wire goes to the Load terminal (middle), and the other terminal goes to ground anywhere at all. I used an empty M6 nutsert ahead of the fuel pumps on the deltabox chassis member for my ground. To be clear, you do not cut off any wires, you simply strip about 1/2" of insulation off the blue wire and the brown/green wire and join your new wires in there with solder and then protective sleeve/heatshrink/elec tape.
Basic pics below: