My 2010 Subaru Forester left tail light DOES NOT work but my brake light DOES work. (its the top light of 3 in the assembly) Both share the same new bulb so I know the bulb is works. Fuses all check out as best as I can tell.
What next?
A circuit problem in the tail light circuit. One thing to check is the integrity of the bulb contacts in the bulb holder. I’ve found corroded ones before, but generally on cars older than yours. I also found a contact once that was bent out of shape and not making contact with the bulb lead. This was after using a multimeter to determine proper voltage getting to the bulb holder with the switch on. That should be the first thing to check.
Since the other side works power is getting to the rear lights but somehow the left side has a problem. As @BustedKnuckles stated, the trouble could be with the bulb socket, with the power lead to it, or the ground for the light. Having at least a test light probe to check for power would be a big help in finding where the trouble is at. It is a very simple problem to fix.
There are two filaments in the one bulb with two contacts so it is like two bulbs in one. I know its a new bulb but just because the brake light portion works, doesn’t mean the tail light portion is still ok. Try another bulb, then freshen up the contact points and the socket and check for any frayed wires.
Be sure too that the bulb is properly oriented and fully seated in the socket.
Remove the bulb and using a DVM measure the resistance of each of the two filaments. If both filaments measure the same resistance, & just a few ohms – like 3 to 10 ohms, then you know at least that the bulb is good.
Try another new bulb. Be sure the bulb socket contacts are not dirty or distorted. With only parking lights on test for power at bulb contacts with insulated probe test light. No power, back trace socket, socket connectors, wire connector and wiring.