check on the picture i attach.
This is only signal ground wire so no requirement for heavy duty switch or wire, it only grounds out the relay signal to activate relay.
The switch I use is the single pole switch , however any SPDT Centre Off switch will do.
You could also just use a Single Pole switch (sp) and just use high speed.
My set-up is use single pole switch and just use the high speed fan .
Ok here is how it works, if you look at the pic of the circuit it shows those two wires connected to the ECM ie pins #14 and #13. If you look at where they are connected they are the grounding circuit of the signal wire for a relay. There is already power connected to the other end ie Pin #14 is connected to pin #2 of cooling fan relay-1. The other end of that relay signal is already connected to 12v via pin #1 of the relay the G/Y wire. So all that is required to complete the circuit is to ground the LG/R wire, that is what you are doing with the switch and that is what the ECM is doing when it receives the signal that the engine requires cooling.
Did I explain that ok? Any time you ground pin 14 or 13 with the ignition turned on you are going to activate the lo or hi cooling fan because you are grounding or completing the circuit to ground that is already connected to 12 volts via the other end of the relay.
(Pin #13 = light green , Pin #14 = light green with red stripe )