They switch an earth, the easiest way to do it would be to use the two outer terminals of the h4 globe to trigger a relay, if you want to put a switch inline, you have to break one of the lines and run it to a switch inside the cab
Run a 6mm cable from the batt +ive via a 30 amp fuse to pin no 30 on the relay.
run two wires (or one 6mm and then split into two 4mm) from pin 87 on the relay to the spotlight for the posative and the earths to a good earth point.
Tap into the two outside pins of the head lights and wire these to pin 85 and 86 of the relay, if you want and should have a isolation switch inside break either the wires to pin 85 or 86.
Using twin core from the relay to switch and back again would be easy.