Rear light and brake light fault

Hey guys. A week ago I had my 2008 mk2 yell at me that the left rear light was out. I stepped out to check, and it was running fine. After driving for a couple of minutes, the fault disappeared. Fastforward to today, right as I went into a carwash it yelled at me again, this time for the left rear brake light. I stepped out, had someone check if they turned on while braking (left one did not, right one did) and later when it turned dark outside I found it it also involved the normal left rear light. The reverse and indicator still work fine.

I've done the CarChris extra ground DIY thingy to both my rear lights so I don't suspect it's the ground that may have gone bad by now since I have a back-up. Any ideas? Thanks!