Brake and left rear turn signal not working

Thanks Ken.
I checked every fuse - okay.
I checked the bulbs- okay.
I check ground at the rear light - okay.
(I used a test light from a known power source or in this case the taillight wire to the ground and it worked)
I searched for diagrams to try and determine where power originated and where it ends in relation to the flasher relay, BCM, fuse block, that I was unsuccessful. I did locate what wire came out of flasher relay, yellow wire.
I exposed the copper wire and ran a new wire from that point to the left rear brake/turn bulb as a test and another ground wire at the rear and turn signal/brake light worked correctly. So I cut the wire at the flasher relay and and ran the new wire to the rear location. At this point all lights are working correctly. Now that I have your post and these images I will look to see if by doing what I did I have bypassed anything.

Not sure if it is okay, to hijack someone else's question but I am having a similar issue. Left rear turn and brake not working. So far I have checked bulb, ground etc at bulb, fuses (under seat as well as under hood) - no signal getting to fuse-box under seat, thought it could only be the flasher module, so bought a new one. Same problem with the new one. Isolated the wires coming out of the module, when I touch a tan wire with my test light the flasher relay starts clicking at normal speed but the light in my test light does not flash. Frankly this is where I hit the wall. What could possibly be causing this to happen? If I thought that cutting the wire and running a new wire back there would work I am ready to do that, but I am hesitant to cut a wire, have it still not work and then have to deal with a cut wire and still no turn signal.

I will try to check the grounds. Although I fail to understand how a bad ground would cause the unusual behavior with the test light.

Well, I figure I need to drop in and tell you what happened, First of all. Suggesting I take a video with my phone and upload it says that you certainly have an awfully high opinion of my computer/smartphone skills. Believe me, I did try.
In the end, I decided to just go ahead and splice a new wire. Certainly something I wanted to avoid as it honestly was my "last resort" option. I went in and unplugged every coupling, starting with the bulb and working my way forward. Testing for a signal as I went. I finally found a signal, re-plugged it. Worked my way back to the light, finding a signal each time. Finally got to the bulb, and it was working. So I guess it was a faulty connection in one of the couplers. I don't know if I should be happy that I got it working, or unhappy because I have no idea what I did to fix it other than to unplug and re-plug stuff.