Thank you for input layback!
When I turn on the emergency flashers, the flashing sound/frequency is normal with the exception of the rear left flasher not working.
After reading a few threads:
http://www.ausjeepoffroad.com/forum/...d.php?t=154903
https://www.jeepforum.com/forum/f310...e-help-986447/
I decided to take out the relay and investigate. With a digital multimeter I checked the continuity between all components, their joins and traces and detected no anomalies. I then decided to "bake" the relay in my oven at 200C for 10 minutes and reflow the soldier. I have successfully used this technique before in fixing computer printed circuit boards that would intermittently fail or cause issues.
Once done I went back to my car, put the relay back in and the problem still persisted. I then decided to take another look at the bulbs and wiring on both sides and with the indicator on, I would wiggle the wires trying to see if there were any dodgy wires or joins that would get the flasher to work, but nothing happened. It is worthwhile to note that the previous owner had a trailer socket installed and the electrical wires were spliced into the rear right lights assembly (for stop, reverse and right indicator) and rear left light assembly (for left indicator).
As I was tiding things up a bit (insulating some joins, adjusting some wires so they don't get squashed...etc, I decided to let the rear left indicator running/flashing and I hadn't even noticed, but some time (10-15 minutes) throughout the process it started flashing normally again. As such, I have NO IDEA what fixed it. When I was pulling and moving the wires before it made no difference. I am wondering if the relay was perhaps "sticking" somehow and "unstuck" itself after running it 10-15 minutes.
This happened on Saturday and on Sunday I drove the car, 24 hours later, and the indicator is working just fine. Beats me to be honest.