Try disconnecting all connectors, spray them with WD40 after wirebrushing the ones you can access.
Unplug all wires going to circuit board and clean / spray.
Unbolt ground connectors, wire brush and add di-electric grease before re-bolting.
As the other fellows mentioned, corrosion could be your problem.
Another thing that happens is called: carbon tracking if you can visualize and imaginary circuit running from one place to another caused by moisture and minerals.
It usually appears in high voltage areas like plug wires, but I have seen it in older cars / trucks.
For example, turn signal causing all lights to flash in dash.
I used to wholesale vehicles and solved a lot of problems with WD 40.
Sometimes wires will develop corrosion under the insulation.
Try peeling back the insulation on a few wires and look for corrosion.
Electricity does some strange things.
I have seen service entry "pig tails" with green creeping crud under the insulation all the way thru the wire.
Battery terminals for example, may look good after cleaning, but if you peel back the plastic molded wires you may get the surprise of a lifetime.
96 Dolphin 36' 454