Simple re-wiring

Take a test lamp, unhook the ground cable and hook the battery in series with the test lamp to ground.

Start pulling fuses one by one.

You have a modern stereo/ radio with "memory keep alive?" How about a separate high power amp?

If pulling the fuses doesn't drop the load, unhook the main alternator output stud. You might have a leaky diode, or the internal radio capacitor (condenser) may be leaky

What do you mean by "ignition switch wire?"