Common with Volvos of that era in general, and specifically 30 and 40 series engines.
Usually wiring harness connector related; check the ones where they connect to the engine and at the instrument panel. For long runs, cables
are often joined together, those connectors can be hidden and hard to find.
The root problem seems to be a combination of undersized wiring, crimped connectors and corrosion
in the connector itself, which leads to overheating
and shorting between conductors within the connector.
There are various workarounds, generally involving finding the bad connector and either jumping the individual conducter(s) or eliminating it entirely by cutting it out and splicing all the wires.