Are you sure it isn't the gears? That is a common problem. The jaw motor goes through the motor board before going to the backbox are you saying the motor board isn't getting 12V?

Q28 is the transistor on the driver board you need to test for the jaw motor

Just to confirm the voltage reading I would test continuity on that wire. Most of the time the break will be at the connector (or a bad connection at the connector). Get a probe with a sharp point so you can punch through the insulation on the wire (you could also use a needle) and test the connection across any connectors.

If the wire is broken someplace in the middle about the only option is to start cutting tie-wraps and looking for the issue.

Not sure what you're asking here. Maybe a photo and more detail.