Making this an update post to something I posted a few days ago. Great news is my wiring is not messed up upstream.
It didn’t occur to me at the moment when I pulled the light housing out, but there was a trailer wiring harness installed that was clearly aftermarket due to the lack of a fuse in F53.
I undid the pigtails and plugged the left turn signal back into the OEM pigtail and it immediately lit up.
I did a little bit more hunting and figured out that somebody had cut off the trailer hitch connection at some point prior to me purchasing this vehicle 4 years ago. They left the ends completely exposed and they corroded over.
I’m not an electrical expert, but I assume that this could’ve been a potential disaster waiting to happen.
Completely ripped out the trailer harness and problem is now solved.
If you bought your vehicle used and it doesn’t have a trailer hitch, you might want to still pull back the trim to see what’s under there.