So my friend sink his 73 Ford F-250 in a pond last winter. Slowly he’s getting it running again. However none of the gauges work. And no lights at all. Where would one start to troubleshoot this type of thing.

 

This can be done with a good test light.
Start at the fuse panel and assure all contacts are clean and work your way up to the light sw. See what power you have coming in and going out when sw is turned on.
DON'T forget. All the power in the world can't do anything without good, clean ground connections.