Headlights dim low, then high?

The logical place to start is by having the charging system tested. If you have a "Volts" gauge on the dash, I suspect you'll see that going up and down with the head light brightness. There's two common ways this can be caused by a failing generator.

You can start the diagnosis yourself with an inexpensive digital voltmeter. Harbor Freight Tools has a perfectly fine one for $7.00. You can find them at any hardware store or Walmart too. This article shows how to use it:

https://www.2carpros.com/articles/how-to-use-a-voltmeter

They're using a meter with the "auto-ranging feature. That's an expensive option you don't need. I can help, if needed, with setting up your meter.

Start by measuring the battery's voltage with the engine not running. It should be very close to 12.6 volts. Next, measure it again with the engine running. Now it must be between 13.75 and 14.75 volts. If it is, that only means it is okay to perform the rest of the tests, but those require a professional load tester. This article shows this test being done:

https://www.2carpros.com/articles/how-to-check-a-car-alternator

They get into the diode test too, but the professional testers check them too.

If you measure the battery voltage while the head lights are fluctuating, tell me what that voltage drops to when the lights dim. If it goes all the way back down to 12.6 volts, the best suspect is worn brushes inside the generator. At the mileage you listed, that's a really good suspect. Those brushes can be replaced separately on many models, if you can find them, otherwise just replace the generator. Worn brushes always start out as an intermittent problem, but it gets worse over the next few months.

A less common problem occurs when one of the internal diodes fails. Those are one-way valves for electrical current flow. All "AC generators" have at least six diodes. When one fails, you will lose exactly two thirds of the generator's current producing capacity. Your generator is rated at 105 amps, so with one failed diode, the most it can develop is very close to 35 amps. That's not enough to meet the demands of the electrical system under all conditions. The battery will have to make up the difference as it slowly runs down over days or weeks. This is where low system voltage could be caused by turning on more lights or fan motors, to the point the engine shuts down.

The professional test starts out with the same charging voltage test you did with the voltmeter. Next is the "full-load output current" test. This is where they're going to find 105 amps, 0 amps, or 35 amps. If all they can get is 35 amps, a diode has failed, but the tester will also show that as high "ripple" voltage. That's the "AC voltage" they're talking about in the article. A few testers can make paper printouts of the results. Those will list ripple voltage as an actual value, but most testers just show it as "high" or "low" with a series of flickering lights. 35 amps and high ripple voltage proves a diode has failed. In most models it is not practical, or an easy job to replace diodes. The more cost-effective solution is to just replace the generator with a professionally rebuilt unit.

Also check out this video:

https://youtu.be/Z-p4dMJUpUw

Here is how to change the alternator out in case it is not charging. If the alternator is okay, (battery warning light off) then I would replace the headlight control relay under the hood in the main fuse panel. Here is the location as well, it should say on the fuse panel lid which one is for the headlamps. Check out the images (below). Please let us know what happens.