There is no circuit board on the E body gauge cluster FYI.

The wire from the sending unit goes from the bulkhead connector straight up to the back of the oil gauge. This wire has a 90 degree push on connector at the oil gauge.
Maybe it came off?

Since grounding the sender wire didn't peg the gauge upscale then we know the sender is not the problem. You could inspect the bulk head connector for a bad connection as Robert said. Maybe ground the oil gauge wire there if you can identify it but that isn't easy in that mess of wires.

Another option, reach up under the dash and feel for the 90 degree connectors (plural) on the back of the oil gauge. Make sure they are pushed on tight. One is from the bulkhead and the other is from the voltage limiter which supplies the 5v power to the gauge.

If all the above is OK I'd pull the 90 degree connector off at the back of oil gauge(the one from the bulkhead) and ground the oil gauge stud momentarily to see if it pegs upscale.