Seems pretty simple
On the drivers side door switch
With the switch off there should be 12 volts (battery voltage) between pins
- 2 to chassis
- 1 to chassis
- and 0 volts pin 1 to 2
With the switch up (maybe down the schematic isn't clear) there should be 12 volts between 1 and 2 and -12 volts with the switch in the other direction (zero neutral position).