Headlight and fans, use a relay
Basic fuse requirements
(from Painless wiring
Electric Fan Relay
Electric Fuel Pump
Radio Ignition (Switched)
The fuse requirement and wire gauge has to take the accessory load into consideration. Using too small of a wire can let a wire burn even though the accessory load matches the fuse rating. Using too 'big' of a fuse can let a wire burn even if the wire gauge matches the load.
Set the meter to the 20 VDC volt scale (or whatever is close to that). Ground the neg. meter lead, then probe the wires separately. In the "at rest" position- brakes not
depressed, there should be power IN but not OUT.
With the brakes depressed there should be power at BOTH terminals.
If there is no power IN, the wire has burned or the fuse blown, or the wire has been disconnected.
If there's power IN to the switch but nothing OUT with the switch activated, you have a bad switch.
You can also test the switch using the ohm setting, be sure to disconnect the switch from the wiring first. Connect the meter leads, one lead to each terminal. If the switch is OK, you should have continuity (close to zero) with the switch activated (brakes ON), and infinity with the switch at rest.
If you have a high ohm reading w/the brakes ON, or infinity, the switch is no good.