I have no idea what your factory wiring should look like, but maybe you could easily test your theory about grounds before getting sucked into diagnostic diagram quicksand?
If you suspect it's a ground issue between the dash and the chassis, would you be comfortable just running a long wire between the negative terminal of the battery and an obvious ground point on the back of your dash? This might be a quick way to test your hypothesis about grounds being the problem.
For example, you could ground just one dash light and see if that fixes it. If it does, then you can continue chasing factory grounds with a little more confidence. If not, then you might have to step up to wading through wiring diagrams for missing power connections or shorts.
I agree this stuff is painful and the factory diagrams are not easy to interpret the first few times. I've had to use both wiring diagrams AND assembly diagrams to figure stuff out and it took a lot of time. Good luck and I hope it turns out to be something simple.
Mike
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