If your right about the 15 Pin Hook up to the Cluster you have problem all right
Some Body Shop books show the Schematic's
1 Way to deal with is to go to a self serve salvage Yard , they are full of early Diesels and just snip off a pin Hook-up with the wires still attached both sides
of the plug
" been their and done this " and that works sometime to at least get the wires correct or check if the match . Try for the same year model you can search online for the car you need a fresh one with the cluster still in

I don't Know why they would disconnect the Pin connection ? perhaps looking for a Hot lead ?
The Ground mentioned is a Good tip check that out if possible