[Airjer_]

You have a corroded/almost broken/poor contact wire somewhere. When there is no load/open circuit on the wire you see twelve volts. as soon as the bulb is installed the circuit is closed and the added resistance of the corrosion before the load (bulb)will drop the current to nothing. With everything plugged in and the bulbs installed back probe the wiring until you find twelve volts. The problem is in between where you found 12 volts and where you had no voltage.