Looks like what I would expect if Gen1 isn't charging. Red wire is voltage sense - it tells the alt what the voltage is at the other end(usually a junction terminal). Brown is the wire from the light. 12v is expected when the key is on but engine not running. Current goes from the Gen1 bulb (12v) into the alt (at low volts) causing the bulb to light. When the motor starts, the voltage on that terminal should go to 12v once the Alt starts charging, balancing the 12v at the bulb and with matching voltages, the bulb goes dark.
Might be time to pull Gen1 and check the brushes. Bad brushes = no rotor current = no charge.