You could open up your old clocks
As unfazed said all the bits not working are fed 12v by the brown wire, you could trace where that runs into the clocks and onto the board, bad solder joint, damaged track on the board, something like that. Should be an easy one to find, and not far in as you have listed all that it feeds except temperature and fuel lights, which wont normally be on anyway.
Not on ground side as you list no neutral or oil light. so definitely the 12v feed side
get you over your mileage problem.