Turning key to "run" causes all power loss.
My 74 Valiant had a faulty ignition switch (it wouldn't send 12V to the starter relay), so we replaced it with a new unit. That one was faulty from the start (you had to hold the key juuuuust right to get the starter to turn on), so we made arrangements for a replacement. I hooked up one of those handheld switches to the relay so that we could use the car. In the meantime, the new faulty switch began to cause other issues. The ammeter stopped working (it always reads 0A). But the car still ran. Then it didn't. The button would still turn on the starter, but the engine would never catch. I found that I had no spark. And only 0.3V to the positive side of the coil while the key was in "run". And then I found that none of the electrical items were working: blower, headlights, etc. I checked 12V at the battery, but only 0.3V on the wire going to the ignition switch. I disconnected the battery and suddenly I had 12V at the ignition switch again. But when I turned the key to "run" all voltage drops to 0.3V. I finally got the new switch and installed it today, assuming it would fix the issue, but it did not. I can let the battery sit for a few minutes, then disconnect it and reconnect it, and I will have 12V everyone again. But as soon as I turn the key to "run", I get nada.
That wasn't good running with a switch that is wrong. Get the '74 manual and compare the switch and pigtails. Also compare with the original.
Assuming your on a computer, you can also use the search function for the forum. Its in the upper right corner of the screen. Search for 74 and 1974 within this sub forum (electrical). Also search for seatbelt interlock. The bypass is easy and done in the engine bay. Either push the button or permanently bypass as you'll find is generally recommended. The complications come with the control unit and the original routing. I suspect disconnecting under the seat may not have played well. I would think that would leave the start circuit open but haven't studied it.
An ammeter at zero means one or two things.
The needle is broken,
its come apart (literally), or no measurable current is flowing through.
It should be zero when a car is running and the battery is fully charged.
It should have shown discharge while starting and charge after starting. Discharge with the engine off and dome light on.
If it has come apart, then there is no connection, or no good connection to the battery.
2. Probing the switch wires. I checked the switch operation. 12V source (red) to yellow and brown on "start". 12V source to black and blue on "run". 2 small red wires run to the key-in-ignition switch and are good. 1 orange wire provides illumination voltage to the lamp on the gear indicator. The new switches came with a small black wire that was not active on any positions. Not sure what this one is for. I taped it off.
Wire 12 ga Red is J1 (Ignition switch feed) It should always be hot.
Do the above search and you should find diagrams for '74 and possible connector drawings.
3. Disconnected fuses. I disconnected 4 of the 7 fuses in the driver's side fuse box. Each fuse passed a continuity check. This still cause a voltage drop. I reinstalled those 4 fuses and removed the other 3 fuses. They likewise passed continuity and I still have the voltage drop issue.
Off the top of my head '74 has nothing critical for running the car fused.
Voltage drops are caused by two things. Current flow rate and resistance. Voltage drop occurs when high current is flowing through some resistance, or low current is flowing through high resistance. V = I x R
Continuity only reveals that current flow is possible (there is a connection, but it may or may not be a good connection)