Welcome to the forum.
Looking at your signature it is obvious you are not afraid to get your hands dirty.
I suggest swapping the tail light assemblies. Yeah, yeah, I know they are not interchangeable on the body but if the electrical connections are identical you can swap them electrically and see if the problem remains on the driver's-side or moves with the tail assembly. If it is the former them it looks like the BCM or wiring harness has an issue, however if it is the latter then the problem is with the tail light assembly and is easily corrected with a new tail light assembly.
Let us know what you find.
By the way, your signature mentions you removed the wind screens. What are the wind screens you are referring to?
2005 SL AWD, Platinum
-OEM HID conversion & HID fogs
-OEM iPod interface
-Upgraded (non-Bose) speakers - Pioneer TS-A1702C & TS-G1643R
-Muth signal mirrors
-Valley Industries hitch and wiring harness
-Aluminum fuel tank shields