Check the actual socket where the bulb plugs into for any corrosion or damage/burned terminals. If that's ok, you probably have a wiring problem or high resistance somewhere along that harness that is killing the voltage to the bulb and making it dim.
GM Dealer Technician For 18+ Years In the automotive industry for 20+
Everybody who reads this post; save yourself lots of time and frustration looking for the problem and tearing your car apart, and just cut off the cable ends of your aged cars battery cables and replace them with a fresh terminal end.
Clean your battery terminals well, with a battery terminal cleaner/brush (any automotive parts store usually has this close to the terminals you'll be buying to repair the ends of your corroded battery cables)
This connection is a critical electrical passageway for current-flow to and from your battery, to the different electrical circuits in your car that depend on this connection or current passageway being 100% healthy Think of it as the electrical starting point**. And when its corroded, under the nice looking rubber encapsulated head where you can't see it hidden like cancer, at the end of the battery cable, things go wrong with the electrical circuits, often in a major way, as-well as in minor ways (different devices in your car don't operate correctly), your lights dim up and down or flicker, weird things happen when you step on the brake, unexplainably, headlights suddenly won't operate and all kinds of other weird electrical phenomena occur!.
So, do yourself a "BIG" favor and start by doing this repair up front if you own a car with these kind of cables and save yourself countless hours tearing your car apart and chasing your tail , looking for the problems.
Take it from a guy who spent over seven years dealing with corrosion on electrical/Avionics gear aboard Naval aircraft everyday and has seen the same problem on many, many cars in his life! May you have good fortune after replacing the corroded cable ends!