Alright, Here's how I see your situation. You have been troubleshooting from all angles at once. Therefore, you're chasing your tail. I'm a service tech by trade, but not on cars. But here is how I'd go about it in this case.
1) Choose one end or the other to start at. (Bulb or battery)
2) Chase your voltage (or ground) toward the other end.
3) You'll find it!
Now if I were to guess where your problem is from my couch, it's behind your fuse box. Pull it out and see if anything sticks out. But I have a few questions first.
1) Do you have 12v on one side of the fuse slot with the fuse out?
If so, I'd guess that everything from the battery to the fuse box is okay.
2) Is the other side of the fuse slot grounded?
If so, then it's time to start chasing that wire. I assume it goes fuse-->brake light switch-->bulb. The tail light wire should come from the headlight switch.
3) Are the tail and brake lights on the same fuse? If so, and both headlight and brake light switches are good, then the issue is between the fuse box and those switches.
I guess what I'm getting at is this. You need to break the circuit down into smaller sections. Check those sections, and find out if they are healthy. I like to use any place something unplugs to isolate sections. For instance, if you unplug the wires at the brake light switch, is the load side of the fuse slot still grounded? If so, your issue is now between the fuse and the brake light switch plug. If not, it's after that. Then just keep working your way back.
For now, have a beer. Tomorrow, have a plan! If you want to talk it through, PM me your # and I'll be happy to give you my opinion on things.