aircon fans as cooling fans

I own a Whitbread trials vehicle which is road legal and remains cool on long runs but when I start laning it gets warm very quickly, I have a pair of ac fans to push the air through the radiator and intercooler, but what size cable do I need to wire them in without causing damage.

going directly to a switch on the dash to be turned on and off manually

Each fan needs to be run with it's own fused supply through a minimum 25 amp relay. 25 or 30 amp thin wall cable should be fine for power to fans. No reason you can't use one switch for pulling both relays 10 amp cable should do that. Or maybe better to use two switches for independent operation. Don't try to power them both from one cable unless you have a good fire extinguisher.

Easy enough and cheap to fit a stat to control them.