Been awhile since I got into mine and I can't find a wiring diagram. Power would be off the ignition key and over to a spade male connector on the back of the display assembly. Ground may or may not be through the mounting screws. As I recall the assembly has a printed wiring board and power is routed around to all the circuits from that.
I wouldn't be suspecting the ground. The oil pressure light as I recall gets a ground to the light via the oil pressure switch when closed. That right there would eliminate the ground problem.
Best I can do.