Headlights dim.

Any corrosion in the lamp socket or socket connection to 12V power leads?

 

Assuming you are not getting cheap bulbs online, some of which are dim, I would suspect the sockets.

While there are ways to check for unintended voltage drops, a good visual inspection of the sockets would be my first step.

 

If they are bad they will likely to be damaged. I would cut them out and splice in new sockets with a longer pre-terminated cable for ease of use.

Another check would be the ground connections. A2 and A3. I would bet on a poor brand or damaged sockets first.

 

A fine brass brush or rotary 'pipe cleaner' is ideal to clean the socket electrical contacts, then apply dielectric grease. If they appear to be heat damaged, that is evidence of corrosion and resistance heating, and the socket is not serviceable. It could also be a cut through the 12V power leads. Squirrels and mice have been known to chew on Toyota (and Honda generator, ask me how I know) wire insulation, sometimes right through the conductors. You could use a DVM to measure voltage while wiggling the harness. If it is intermittent, follow the leads visually to inspect them for cuts.

 

Any kind of wire brush is not good because it can leave "splinters" inside the socket and cause a short.
And recommending dielectric grease in places like that is not good either because most varieties of it is an insulator and may cause more problems than it solves.

 

Some voltage measurements need to be taken.
This could be a sign that there is a problem developing in the 12 V power system; battery, main connections or charging circuitry.

 

Are the bulbs LED?
Are you sure the plugs are pushed in all of the way and click?

 

It's possible that you have a poor ground connection at one of the headlights.

 

That's why you use a brass brush, and an air duster following the socket cleaning.

Dielectric grease is specifically designed to seal without increasing resistance on properly mating surfaces.

 

NO IT IS NOT.
There are different kinds and some, the original formulas, are almost perfect INSULATORS.
If you are going to put it on a conducting surface, you NEED to be sure and get the kind that IS conductive.......or else you are likely to be sorry.