Just locate the high beam lamp in your actual headlight and solder onto that for your A1 relay coil wire. You'll need to use a multimeter to find out which is positive (but generally it's the one that's NOT black). If you plan to install a switch inside your cab, you'll need to run said coil wire (probably a 2 core cable) into the cab from the high beam, and then from the load side of the switch into the engine bay to activate the relay coil. I can do up a wiring diagram for you if you like