The map light is part of the curtesy light circuit that can include the console and dome lights. with that circuit, all the lamps have a live positive feed connected all the time. when you open the door, use the dimmer switch or flip the map light switch, what you are doing is grounding the circuit to the body. they are all tied together. This is why when you open the door the map light comes on. The map light has a switch that has its own ground so that you can turn it on while the other lights stay off. Pink energizes the switch and yellow is ground. if you remove the fixture with the two screws that hold it to the dash it is very easy to test for continuity with a circuit tester light or volt/ohm meter while it is still attached.