Car from years ago - Its cabling between the Alternator and Battery had multiple connectors, and they corroded over time, the symptoms I got were that the car would run fine in daylight (unless it rained and I operated the windshield wipers, or lights) and not long at night, the Battery would run out of power. The reason for the problem is this - Say the wiring between Alternator and Battery has just 1 Ohm of resistance - The Alternator cannot charge the Battery at more than maybe 10 Amps, and that's including the load of the car's electrical demands, due to Ohm's Law (i.e. 20 Amps for the headlights, 10 Amps in from the Alternator, this is not good!) as at 10 Amps that 1 Ohm resistance would mean a 10 Volt drop in that wire, so the Alternator has to generate 20V at 10A instead of 14.5V at 60A - Meanwhile that bad connector is getting pretty warm, corroding more, etc. so it goes to 2 Ohms then 10, and so on; Pretty soon you're stranded with a dead battery.
Debugging it was maddening - I removed the Battery and Alternator and took them in to have them tested, "Why won't this Alternator charge the Battery?" and both tested perfectly normal and functional. Had me pulling hair out till I got it!
So my suggestion aligns with Jason C.'s, for safety disconnect the ground lead on the battery and THEN test the cable connections for resistance with a Voltmeter; If you find a high resistance joint, repair however works for you (I bypassed the high resistance joints with a heavy #4 AWG cable directly from the battery to the alternator, as the manufacturer had put about 12 connectors in there for who knows WHAT reason and I lived out in the sticks - and just needed it running right. I'm an electronics guy tho.)
Edit: Re-read and thought, is it possible that the wire from the ignition switch to the starter solenoid, is failing somehow? Unsure if on that vehicle there's a relay on the firewall / inside the engine compartment, Ford USED to do that a lot, you could test by jumping that with a heavy wire to Battery + at the relay; If not you can usually short the solenoid on the starter to +12 in some fashion, just don't rig yourself up so the car runs you over or the like! If the starter is fine but the key won't turn it over, it will either be the key switch or the wire to the relay or solenoid, or that relay or solenoid. That begs the question of what failed and shut the car down though, so definitely look for the cause of that!