The back left bulb in my 2008 Ford Falcon is not working

It seems a little odd that a new bulb hasn't fixed your problem, especially as the other tail-light is working properly. If both tail-lights had failed, the first thing to check would be the fuse or relay that controls that circuit. But since it's only the light on one side that's affected, that probably won't be the case (both the BF Falcon's tail-lights share a single fuse).

The BF Falcon launched in 2008 was the first local Ford to use CAN BUS electronic technology and that could be the cause of your problem. If the computer isn't sending the correct, coded signal (as opposed of just a stream of volts) to the component in question (in this case the tail-light) then the light may not work. A scan of the vehicle might throw up some more clues.

If it's not that, that leaves you with a bad connection (possibly a bad earth) within the wiring for the troublesome light or perhaps even a problem with the bayonet mount into which the globe clips. Don't rule out something as unlikely as the new globe being faulty, either. It's happened before. Spray some terminal cleaner into the globe mount and see if that restores power to the light.

Fords of this era are notorious for electrical problems, including brake-light switches that fail and even body computers that crash, taking many functions including the central locking with them.