If you have access to the sender on the tank them you could try grounding it there and have someone see if the needle jumps. You've probably figured out it's a hoor to get to in the bay bus. Also try grounding it at the gauge end, if it works there and not the other end you've got a break in the wire somewhere.
Made up a test wire and grounded it off a few different places around the dash and the instrument cluster, no movement on the gauge unfortunately.
I'm beginning to suspect someone, possibly me, wired the gauge up incorrectly in the past. When I was looking at it at the weekend I noticed the 12v supply for the regulator was connected to the gauge, and the wire from the sender was connected to the regulator
This probably toasted the gauge as it's not designed to get the full whack from the van's electrical system (I believe this is what the regulator is for, a primitive surge protector).