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I'd be inclined to separate, clean (as necessary) , and reseat the main engine harness to dash connection....
ground connection - I'd start with the main battery wire - should be around the flywheel cover (bell housing)...
FWIW, looking at electrical connections don't get you much....I prefer to check each connection for voltage drop....usually with one meter wire on the battery....
On the no start- assuming the starter is spinning the engine, I'd check the voltage at the + side of the coil first...if its not spinning (coupled with the gauge issue) I'd suspect you don't have a good supply to the key...that would hint at the breaker or the wiring harness...