The speedometer on my 92 XJS Convertible is inoperative. I removed the cluster today and found that the harness that plugs into the back has a broken fastener. The one on the bottom, where the #10 contact is located.
what do I do? There are about 20 wires that go to that particular plug.

 

I would add wires to each point on the cluster and add new connectors to both ends. cable tie the cluster side of the harness to the cluster so the wires can be pulled out.

Each point on the cluster has screws so you can crimp eyelets to the wires and add them directly to the gauges. This renders the cluster traces unused.

 

I bought a new one

https://www.jaguarclassicparts.com/u...nstrument-pack

Sorry I thought you meant this. ...

 

My instrument cluster is fine. The wire harness that plugs into it is damaged.

 

Yep I realise that. You have 2 options find a set of good connectors at a junk yard and splice them in or make a new wiring harness for the cluster and replace the connectors with good automotive units and never touch it again. The OEM connector setup is dodgy at best and quite unreliable.