Connect your positive wire from fan to battery + terminal and run the ground wire into cab of vehicle and attach to one side of the toggle, the other side of the toggle will be wired to a body ground screw. When you flip the switch, it'll complete the circuit and turn the fan on. Good luck and have fun!

You might save a lot of trouble by replacing the fan relay that has gone bad. If you must do it, wire it series from the battery positive to the switch to the fan power wire. Put the switch where you need it to be. The fan has the same ground and you don't need to add another one.

With respect it's not wise to do this. The thermostatic switch in the cooling system does the job better than you can.

tee into the source wire