I'm not using a switch.
You can mount the switch anywhere you like.
As far as what type of switch, any two position switch, that is rated for over 12 volts and at least a 10 amp rating.
A few here have posted how to install it, and it's so simple it makes perfect sense.
The only thing the switch is doing is providing a complete circuit, via a ground, to the fan motor, since the switch is in front of the OEM's thermo/switch. That's why it doesn't negate the automatic feature of the thermoswitch to also kick the fan on.
All this is for SP1 only.
SP2's require a little different wiring, but still very easy.