First post here. So my left taillight won't work but the brake light is working (same light bulb) everything else on the car works and has no other issues. Anyone know what the problem could be?

 

Have you check the bulb? The bulb has two filaments; one for the brake light and another for the taillight. The taillight filament may be broken.

 

Yeah I changed the bulb about a week ago, is there a certain way to check the filament?

 

Visually check if the two filaments are okay. You can also swap bulbs from the different side or try another bulb.

Otherwise, if you have a voltmeter, test for power on the Red wire at the connector with the headlight switch turned on.

 

Since it didn't come back with a new bulb, another possibility is the contacts down inside the socket.

After checking the wire like redbull said, if that's getting power you can also look for screwed-up contacts inside the socket (bent? corroded?).