It may well be multiple causes.
Fuses would be a good place to start. It could be corrosion on the contacts or a bad fuse. The firewall connector could have issues.
Perhaps you have had rodent activity? Are all the wires intact?
Bad grounds (corrosion again) could be the problem.
When a vehicle sits for a time, sometimes the float in the fuel tank fails and develops a leak (fills with fuel and sinks).
The voltage-limiter on the instrument cluster may be marginal or have failed. Might be time to replace the limiter with a solid-state replacement?
One thing at a time, you can usually concentrate on problem solving each issue.