Guages not working

Which way I would approach such a mess... I would buy a factory service manual and start studying the wiring diagrams while searching for a good used harness.
Once that harness is in hand the internet can help with minor changes/upgrades to prevent potential failures that are known common. Document changes to suppliment the manual, get happy with the plug and play replacement harness, and then remove the original.
Even without another harness the manual and the workbench are a must.
Working upside down under a dash usually turns out about like what you already have.