SB;

122 electrical systems are thankfully quite simple, so unless you are troubleshooting the charging system (which is a lot trickier) your faultfinding should be a simple matter of tracing with a voltmeter or "test-light", from the power source, to the controlling switch, to the load...and don't forget...in a vehicle, completing the current path (to the negative side of the battery) is often some sort of connection to the body/chassis (which over the years may have become crusty and green, and is by now a poor conductor), and the sheetmetal back to the battery negative strap.

"Common trouble-spots" are any place where there is a connector or connection...wire rarely breaks en-route! Sorry to be so vague, but that's the way it is...ANY connection can be suspect on a 40 year old vehicle!...so when you've located the offending connection(s) using step by-step troubleshooting techniques, I always recommend cleaning both contact surfaces with an abrasive (wire brush, emery cloth, sand off the road in a pinch) then applying anti-corrosive paste when reconnecting...Done!

If the interior light is not working, you probably could start by cleaning up the conical ends of Fuse 4, unless you have a headlight flasher relay, it's the only thing powered by that fuse.

Suggested reading: http://www.intelab.com/swem/an_automotive_electrical_primer.htm
and: http://www.intelab.com/swem/gastight.htm
and: http://www.intelab.com/swem/anti_corrosive_paste.htm

Good Hunting!