I had the same problem when I went to wire some spotlights to my 99 Jimny, none of the wires would be live at the right time (in fact one of them did the exact opposite of what I needed it to do).
The wiring all seems to be floating above earth and goes from the fuse box through the accessory or light then through the switch to earth - I am not an auto electrician (I am a household electrician by trade) but that just seems a weird way of doing it (especially when compared to old cars).
The solution I came up with was to wire a small relay coil in parallel with the high beam indicator light on the dashboard and use that relay to switch power via to what you called the master switch (which in turn powers the 15 amp spotlight relay).
If you look behind the removable panel at the base of the steering column there is a connector with 16 wires in it (or at least there is on my 1999 model).
At this plug I tapped into the white wire with blue stripe (which comes from the fuse box) and the red wire (which comes from the headlight high beam lamp and goes to the headlight switch) to power a small relay coil.
I then connected the same white/blue wire to the relay contacts to switch power the master switch.
Because this modification is in parallel with the high beam indicator light it even works when the headlights are off and you use the 'pass' switch.
I hope this makes sense, happy wiring