Through 1971 there were 2 switched hot wires going to all instrument panels. A blue with white tracer from ignition switch fed the instrument voltage regulator and the system voltage regulator so both regulators were on the same circuit. The other switched hot wire was black and fed park brake lamp and oil warning lamp. In 1972 Dodge deleted that blue with white tracer routed to instrument panel. The inst' voltage regulator is now on the same black wire as park brake lamp, oil warning lamp and maybe a seat belt warning lamp also. We might assume Plymouth is the same. Bottom line... you'll need factory wiring diagrams for your 73 model Plymouth.
A 3 amp fuse should be for the inst' lighting and only hot when light switch is pulled to park lamps. There is a way to install a replacement ignition switch wrong and blow this fuse. If the switch has been replaced and orange wire gets attached to a black wire... there is your problem. Good luck with it.
