Why did my headlights kill my car?

[Mattax]

Where is this circuit breaker located?

In the headlight switch.
There is one self-resetting 15 amp breaker.

Then they began to flicker, so I pulled over. As I pulled over, the entire car went dead.

I also had a starter relay failure at about the same time as the total electrical failure. I replaced the starter relay yesterday. Both the battery positive cable and the main electrical feed from the battery go through the starter relay. I wonder if somehow the failure of the starter relay had something to do with this.

The fact that car had lost electrical power when the engine rpm dropped low indicated that neither the alternator or battery could supply enough power to the main splice.
Therefore yes, loose or othewise poor connections in the wires connecting at the starter relay could be a culprit. Additionally, the alternator output could be poor, perhaps at its connections (there are only two). With 14 Volts at the battery while idling might seem to rule out poor alternator output but worth checking a little more precisely. When doing so, be sure to check the ammeter reading (dash gage labled alternator.)

 

[Mattax]

Both the battery positive cable and the main electrical feed from the battery go through the starter relay.

They make a junction there.
If the car has a rear window electric defrost grid, it probably has an optional HD alternator and wiring.
Otherwise power feeds are set up like this.

The ammeter shows how much electricty is flowing in or out of the battery.
During normal driving even with lights on, there should be no discharge out of the battery.
At slow idle with lights and brakes on, it may discharge slightly.
If it does, then it should charge for a minute or two once the rpms come up to even slow driving speeds.

When you see other conditions on the ammeter, they can be a big clue as to whether there is an a failing alternator supply, an open circuit, or a short, etc.

 

[Dartswinger70]

It looks like thet wroe from the headlight swithc to the main feed might be where to look? or the switch itself...then you have the wire tothe distributor

 

[Bewy]

I think you have a short in the headlight cct. A bad h/light ground does not kill the entire electrical system.
I had a strange problem in my Dart years ago. HLs would get dim after wet weather. OK in dry weather. WTF?
The problem: before I got the car it had had a front end collision repair. The HL wires had been squashed between the rad support & bumper apron. The dust/pollution that settled there became conductive after getting damp & created a resistance short to ground; which slowly discharged the battery because the alt could not keep up with the current drain.