Take apart. Or remove if removable. Order new if removable, or get replacement from PGDT and/or fix it if not.
I might get a cable on ebay or fix it as not cheap!
This shows the permobil joystick (he thinks its a smaller version of the CJSM2 but its really branded and made for permobil alone) has a removable cable. But he may be wrong. I saw a removable one once and I am sure it wasnt permobil but a real CJSM2 but I could be wrong, wasnt paying attention.
If its not removable, then it actually still is, you just need a soldering iron. Only 4 wires not hard to do. Well I didnt actually do it, but I have seen it internally. Should be easy enough.
https://www.youtube.com/watch?v=hQrwEItrP_w