Left brake light not working sometimes

[D-Day]

It's being funky as hell. I got stopped by a cop the other day and he told me about it, but he didn't ticket me because the light wasn't out. The 4-ways work, the left turn signal works, just when the brake is down it sometimes didn't work.

Then it started working again.

Then it started malfunctioning again.

It seems like when it's not working, if I flip the left turn signal, it'll start working again. Something is amiss here. Anybody have any ideas on where to look for the short/wiring problem? It's not the bulb.