Well, I'm totally baffled. I was wrong, there is a circuit board back there. (I should know not to trust my memory anymore). But I checked the left side against the functioning right side, and all voltages and resistances are identical. There IS voltage on the left side and it is changing with the flasher pulses just like on the right side. The brake light on the left side still works so there is a ground. The flasher works on the right but not the left, which makes sense. I tried switching the right and left bulbs side to side, and no difference. I tried switching the twist in sockets that hold the bulbs side to side and no difference, so it's not the bulb holder/socket or the bulb. How can there be voltage and the bulb doesn't flash??