Each door jamb switch is DPST shunted to ground. The key warning alarm connects to one pole of the driver side (left) door jamb switch as a standard feature, so that the key warning alarm will function. The courtesy/dome lamp connects to the remaining pole of the left door switch, and to one pole of the right door switch, whenever cab entry illumination is purchased as an option. Hence, in the standard configuration, only the left door switch is installed from the factory. The two poles of a door jamb switch are electrically isolated from one another while the switch is open (door closed). Both poles complete to ground when the switch closes (door opens).
The way your vehicle presently is configured, only the key warning alarm connects to the left door switch. The unused pole of the switch is "available" to connect the courtesy/dome lamps.
When cab entry illumination is installed, the two door jamb switches are electrically wired in parallel with the courtesy/dome lamp switch that is integral to the headlamp switch. This way the courtesy/dome lamps illuminate no matter which door is opened or when the dash lamp rheostat is rotated fully CCW.
Cab entry illumination is wired opposite to most other electrical appliances. They operate by switching the ground path instead of the power supply. That is, the courtesy/dome lamps receive constant battery power - power to the courtesy/dome lamps is not switched ON/OFF. They illuminate only when their circuits are completed to ground through the door jamb switches or the dome override of the headlamp switch. The "white" circuit coming from the courtesy/dome lamps are electrically "live" until either the headlamp switch knob is rotated fully CCW to detent or, when cab entry illumination is installed, until either door is opened, at which time the white circuit is grounded and drops to zero.