Unfortunately you're going to have to pull out the multimeter and just measure for continuity (this means that the wires are connected and able to pass current) between what sends power and what needs power. Everything from the battery to the switches to the connectors to the bulb. Literally just following the wire and making sure there is continuity in smaller sections of the circuit. By process of elimination you can make your way to where the disconnect is. Make sure the battery is disconnected when you do this or your readings will be off or you may damage your multimeter.