The CLPSSM ( central locking pump / security_system ) senses the door switches on pins :
16 - right door
14 - left door
( the CLPSSM is the thing buried under the trunk floor and wrapped in thick foam rubber )

It, in turn, communicates with the BCM ( body control module ) via a twisted pair CAN BUS.
The BCM then determines if a ground should be applied to pin 18 that connects to
pin 3 of the interior light switch ( the center position ) - Brown wire.
Fuse 9 supplies the B+ to the dome light and cargo light.

( The BCM is in the box by the battery, closest to the battery )