Stopped cranking

12.3 volts means the battery is close to fully discharged, but it should still run the head lights. First charge the battery at a slow rate for an hour or two, then try to crank the engine. Assuming everything is still dead, follow the smaller battery positive wire to the under-hood fuse box, and be sure that connection is clean and tight. Follow the smaller battery negative wire to the body and be sure that one is not rusty.

Also don't overlook loose or corroded battery cable connections. Turn the head lights on so current is trying to flow. Measure the battery voltage right on its posts, then move the meter's probes to the cable clamps. You should get exactly the same voltage at both places. If a cable clamp is loose or corroded, the second voltage will be lower.

Did you check the fuse box connection?

It's possible a '90 model didn't have that yet. In that case, with multiple circuits turned on, measure the voltages on various fuses under the hood. Circuits that are turned off may have 0 volts, but I'm interested in those that should have full battery voltage. Tell me what you find for voltage on those.