Head light not working.

It would not be the first new bulb that was no good that I have seen. Check everything with a volt meter to make sure that you don't have a bad "new" bulb.

Since the low beam works, wouldn't that rule out the ground (-)?

This must be a very very old rig. 4 rectangular headlights?

The way the OP phrased the question threw me, Thanks for pointing it out.

Since the low beam works, wouldn't that rule out the ground (-)? Only one way to find out. Then again, the prong inside the bulb connection could need a little cleaning.

You need to test not guess.
A cheap test light probe. Available at any parts store.

Remove the electrical plug to the rear of the lamp
The cord of the test probe has an alligator clip. It needs to connect to ground
Find a ground and then shine it up with rough sandpaper
Beware of selecting metal not connected to the frame, or the test won't work.

Switch on the headlights. Verify the other headlamp is lit.

Stick the needle probe into all three terminals on the headlight wiring PLUG

No light? Advance to wiring diagnostics.

If the test light lights up, you have a ground wire problem

Note which wire the test lamp lights up on

Switch dimmer switch to other beam. Note the other headlight

Does the test light now light up on a second terminal of the headlight plug?

The REMAINING terminal is the ground and it's connection to ground is damaged.

booten wrote:
We have 1 high beam headlight not working. We replaced the bulb, checked the fuses. What else could it be?

Maybe your Harley has only one headlight. :W

Wiring to the headlight would be my guess, that includes the ground.