Hey mate, it has been an issue on some Tenere's. There was a post somewhere on here about opening the speedo up but sorry I can't seem to find it but as you have found out it is almost impossible. Found this though so you don't feel so alone with it..

https://www.xt660.com/showthread.php?t=25431

Might just be worth checking that the electrical connections are clean and in good order though.