[alantf]

Now that he's found tail light +ve & ground, that only leaves one more to find - the brake light +ve -. Since the tail lght works, we can assume that the original ground is ok. The fact that he's getting millivolts from the other wire (which can only be the brake light +ve) suggests a broken wire or blown fuse. As he says that the other things associated with the fuse are working fine, that SEEMS to leave only the broken wire. It's possible that it's something to do with when he ripped the old fitting out.