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.
we found a bad relay.
I have an 2005 and the headlights on it will dim, and then brighten back to normal. I bought it brand new, and it has done it since I bought it. I actually took it back to the dealership, and asked them about it, and they said that Chevy knew that it would do that, but that the car would keep running, and that there was nothing that they could do about it. Hope this helps.
Your battery is weak, or your your alternator is not charging battery.
Check the grounds for the headlights.
Usually dim headlights are caused by a poor ground connection.
the courteous time to dim your headlights is as soon as you see another vehicle, or within one half mile.
A bad battery or faulty battery connection.
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.
If by "dim headlights" you mean low-beams... the bulbs are bad and must be replaced.