One tail light does not work

If the running light works but is dim, have someone hit the brake. If the dim light goes out, you’ve got a bad ground. Check the metallic connection from the bulb through the socket into the body or wiring. Rust right at the socket is pretty common, scrub it shiny, put a light smear of grease to suppress later rust. You can try taking a piece of wire, hold one end to the socket contact and find bare metal with the other. A dim running light will brighten up to normal confirming bad ground.