I just gotta ask one more stupid question, you did the speed test with the car moving, right? It will show 0v at a stop.

If it's showing 0v while moving, then you've found the problem, which is that the OBC isn't getting the speed reading. If you're sure the wire is good from the cluster, then I guess you're stuck with something being wrong in the cluster itself. I'd start by replacing the famous capacitors since they're a known issue (and cheap), resolder any bad-looking connections on the board, and then worry about detailed troubleshooting of the board afterwards if that doesn't fix it.