Driver side brake light does not work?

Insure both side of the equation are clean (do this with the power off) I mean bulb and socket. Using dielectric grease is a plus also if that is no help run a jumper wire from a good ground--i prefer all the way back to the battery's neg post I have a good many long jumper w/ alligator clips on both ends---i get stuck troubleshooting wiring on utility trailers and boat trailers anyway lets attach the end opposite of the battery to metal adjacent to the socket (if the metal is part of the sockets ground)

push a "straight pin"--"needle"--"hat pin"into the ground wire as close to the socket as possible. Then attach your alligator clip on to it (the pin stuck into the copper wire) this will insure you have a ground to test by----if it all works fine, you are gonna have to trace down where the original ground is supposed to hook to the body or frame and repair/ clean/ reattach it many times the bulbs are dim--even make the others alternate flashing dimlyusually its a bad ground, and the only reason you see the dim lights is because it's seeking a ground thru the other turn signal or running light wire

keep us posted on progress

Passenger coming on-check the yellow wire on the bulb socket for the driver side- no power there-investigate the turn signal switch