If the board has been checked, then the issue sounds like its with the plug/connector.
I think from memory this is J106 on WPC96?
(bottom left)
I'd start with some easy testing first:
Unplug the playfield GI (J105), put the Backbox connector into J105. Does it light? That would confirm whether or not the issue is with the power driver board or not.
if it remains unlit, I would check the header. Its likely either a cable (or cables) have become loose or have snapped.
This photo is of a WPC89 board but the inspection is the same.
You want to see a clean connection between each pin on the IDC header. See how each cable has effectively 'teeth' biting into the sheeth? That's how its making contact.
Here in this second picture is a bad wiring example, notice how the cables look pretty chewed up. This is when they've been reseated too many times and the sheeth is pretty much destroyed, leaving a loose or broken connection.
Ideally you need an IDC punch tool.
IDC Pusher / Wire Insertion Tool 0.100" (2.54mm)
www.pinballshop.nl
I'm not sure of a cheaper one in the UK. You could try using a flat head screw driver but honestly it may end up like the second picture!
To repin its pretty simple.
Take a picture before you start to ensure you don't put a wire back in the bad spot!
Cut back the bad end, place back into the plug. Use the insertion tool to press down onto the wire, the 'teeth' will cut into the wire securing it to the plug. Rinse and repeat for every bad wire you find.
Be sure to do this with the machine OFF, the plug AWAY from the power driver board. You can use the bottom of the backbox wood as a temporary work bench to handle the pressure when re-pinning.
The alternative is to completely repin the headerr into a Molex style header but thats going to involve a new plug, new pins and a crimper. Its a lot of work!
Take a picture of the plug and share on this thread if you can.
Good luck!
Scott