1993 Toyota Camry electrical glitch

Those are the classic symptoms of a bad connection. Turn on some things like the head lights or heater fan so the problem is occurring, then start at the battery with your voltmeter, then move the probes to the cable clamps, then down to the next points to see where you lose that voltage. A test light is faster, and in this case, it can be more accurate, but the voltmeter will work fine too. If you have a smaller positive battery cable bolted to an under-hood fuse box, that is a common place to find a loose or dirty connection.