I understand now why you connected to the J1.
S2 is only hot when the key is in start.
I would think the easiest place to draw constant power would be the fuse box. The Q3 feed goes to a two fuse bus. You may find an extra terminal on the buss that is open. But what you did should be OK too.
View attachment 1716410948
With the key in run and the radio disconnected, the only things drawing power should be the ignition (points closed or an ECU) and the alternator field. Since power is coming from the battery, the ammeter should show slight discharge. Something around 5 amps.
Those voltage readings indicate
the alternator output wire isn't connected,
there is some resistance between the junction on the starter relay and the ammeter
the voltage regulator has high resistance (at 11 volts it should be full fielding)
Lets put these on the diagram.
Voltage only drops when current flows through resistance.
We know approximately the current with key in run.
View attachment 1716410953
With only 5 amps flowing through, dropping 1 Volt between the starter relay and the ammeter indicates some resistance.
If the readings are accurate, that resistance is probably in one of the connections. Less likely, it could also be a damaged fusible link.
There is no drop between the ammeter and the voltage regulator, so that's good.
But then there is a 7 volt drop through the regulator. That's no good. If its a electro-mechanical regulator, you can open them up and see if something is wrong. There's a couple fusible links - one might might be burned out, or something is keeping the arm with the points from completing the direct feed.
The other measurement that is troubling is at the alternator's Batt terminal. That should have read 10 Volts. So that indicates a break in the line. Most likely at the firewall.