left taillight wont work

I made a post earlier about a car I'm trying to fix up. right now I'm trying to get the left taillight to work. And I have to add that I've never done electrical work before.

I might have shorted something out when I was testing for a bad ground. The left taillight assembly was acting funny and I hooked the ground wire to the body and blew a fuse. I replaced the fuse and the lights on the right assembly started working again. And when I wedged some copper wire into the spot where the assembly connects to the wire harness the lights on the assembly started working correctly. All of them except the taillight itself. It wont turn on now. I checked the wire with my multimeter near the spot where it connected to the assembly and it had no voltage ( technically .4 but nowhere near enough to make it work) I tested the wire again near the spot where it came out of the harness at the fusebox. Same problem. So I know it's not the bulb, not the assembly, and not the wire.

Any ideas what it could be?