a few years ago both of my front bulb sockets corroded out, but that was the front, theyre exposed, the rear that wouldnt be a common. both bulbs went out? otherwise id start w/grounds 1st. when i replaced the sockets the connectors were important(-0-voltdrop)(i used std crimp connectors, like you can find in the store).
According to what I see in the Bentley wiring schematic, there's one wire that carries juice from the turn signal switch to both rear turn signals, at some point it branches out (unfortunately the Bentley doesn't say where exactly, but I suspect it's somewhere in the back) to each individual turn signal. This tells me that your switch and the wiring for the left signal are all good, and your problem lies somewhere in between the point where that wire branches and the turn signal. If it were me that's what I'd check first.