My brake light functions but is dim?

The most common cause of a dim bulb is high resistance in the circuit that is causing the bulb to not illuminate completely.

Here is a guide that will help with this:

https://www.2carpros.com/articles/one-headlight-is-dim-or-dull

However, within this issue itself the leading cause of it is corrosion on the socket. So more than likely we need to replace the socket in order to restore the brightness.

Here is an excellent video by Ryan that shows how to do this. He takes you through how to solder in a new socket so that it is done properly.

https://www.youtube.com/watch?v=PxA5wczsCVo

Please run through this info and let us know what questions you have.