So I turned on my car on day (1969 Mercury Montego) and suddenly a ton of things weren’t working. The AC, Turn Signals, Radio, Parking Lights, Back-up lights, Gas Gauge. Things like that. I replaced a few of the fuses to no success, and am currently at a stand still for what could be going on.
There may be fusible links between the positive battery clamp and the rest of the system. One may have failed (melted inside) because of a short circuit.
The ignition switch may be defective. When you turn the key to the start position, only the ignition and starter motor receive power. When the engine starts, the other circuits are switched on. My guess is that the ignition switch isn’t allowing the circuits to receive power.
On older cars with electrical problems my go-to is always the ground connections. Battery to chassis to engine to accessory grounds all over the car. Good Luck