Head light not working on high beam
When dealing with wiring on a vehicle, it can go from something real simple, to something real complicated, in a hurry!
1.Checked to make sure the headlights have high beam? Bulbs are good in that respect? Not trying to insult your intelligence, but sometimes it's the simple things we overlook. Again, not trying to sound 'Uppity', but do you have access to a type of jumper wire, that has a needle type of tip on both ends? This way you can stick the needle tip, into the wire that has 'juice' for the low beam side of the headlight, and touch the high beam side. See if the high beam comes on.
2.Sometimes it's as simple as a bad ground. There is a wire, or wires, that come out of the headlight wire loom, and have a rounded metal terminal end. A machine screw, goes through the hole in this rounded metal terminal end, and fastens it to the metal sheet metal of the body.
The metal frame of a vehicle, and the metal sheet metal, is the ground for a vehicle.
A lot of times, where this rounded metal terminal end is fastened to the sheet metal, (Or frame), rust can develop between them. Between the metal terminal end, and the sheet metal. This makes a bad contact for grounding.
There is enough of a ground source for low beam headlights, but not enough for the additional draw of the high beams.
3.In the headlight wire loom, there is a Fusible Link. This looks like a 1, or 2, or 3 inch large bump in the wire. This fusible link is designed to melt inside, and break contact, should it need to. Sometimes it's readily apparent when you view this fusible link. It will have the plastic covering melted on one side, or partially around. Sometimes it's not so obvious. (Isn't wiring fun(?)
You just have to use the probes, (Needle tip is real nice here), on a multimeter, (Set to DC voltage, and the 50 volt scale), and track from battery to headlight switch, to headlights.
4.Lastly, it may be the headlight switch itself.