You could have a short in your electrical system, your battery might need a charge or be replaced.
you might have a short in your alternator. In my car the headlights would dim when I put on the brakes or tried to use cigerette lighter to power up my phone.
Usually dim headlights are caused by a poor ground connection.
ldr sensor
the courteous time to dim your headlights is as soon as you see another vehicle, or within one half mile.
when there are oncoming cars, it is appropriate to dim your headlights when they are about 300ft away. Check your diver manual for the most correct answer.
Your headlights may not dim because you don't have dimmer lights. Dimmer lights can blow and leave you with just the high beams.
You should dim your headlights within 500 feet of the car in front of you. The rule of thumb is, if you can see another car, dim your lights.
If you can see the area lit by their headlights, then you are probably close enough to dim your own.
you may either have a weak battery or your alternator could not be producing enough volts. Could also be a combination of the two.
If by "dim headlights" you mean low-beams... the bulbs are bad and must be replaced.
The headlights on the 2002 Toyota Tacoma might work on dim but not on high beam because of a blown fuse. If the headlights are too dim to see properly in the dark, there could also be a problem with the alternator and the charging system in the vehicle.
They are probably dim because the plastic lens has faded. You can clean them with a product called CrystaLenz. I use this product when I detail headlights at used car dealerships, and it works extremely well. Check out their website www.CrystaLenz.com for more information.