All the + power for EVERYTHING except the big heavy cable to the starter for the starter motor power goes through the firewall bulkhead connector and then through the AMMETER on your gauge cluster before going to everything else in the car including your ignition switch. It is real common for the bulhead connection to get a bad connection and or FRY, and it is even more common for the AMMETER to fail. The Ammeter in these cars is in series with everything so when it develops a problem it affects power to everything.

Follow the positive wire from the battery to the starter relay to the bulkhead through to the other side of bulkhead to the ammeter etc with your voltmeter. Its tough but you can reach up under dash and see what kind of voltage you have at each side of ammeter. I bet somewhere in there you see a sudden large drop in voltage.