One of my car headlights has become very dim. Possible causes?

Check the wire connection, if it is stable, yup you need a new bulb!

I have been successful several times in giving the headlight a good smack. Don't know why, but it got brighter.

its all about the FONZ.

is there dirt across the plastic covering? i'm serious. just washing off my parents' car and focusing on the headlights fixed the "dim lights" problem : )

typically dim lights anywhere on a car is due to a poor ground which can be caused by corroded terminals, broken wire, loose wire. If you know how to remove the light bulb, then get some electronic contact cleaner, it is in a spray, and clean the male and female contacts. also make sure the bulb wire plug (usually a three prong plastic female plug) is firmly pushed onto the male contacts of the bulb. Some cars the male and female are reversed.

it could very well be a bad ground for that headlight but i would also check your
fuse for that light if it is blown it can cause a sneak circuit which will cause it to be dim. check you left headlight fuse.