So I got around to fixing the door jamb switch on a 2003 Honda Civic
LX. I think since it was sold new, the little plunger switch was
never properly connected. There's this rubber/metal cover that
protects it from the outside. The switch has what looks to be a
ground connection via three prongs that plug into a metal plate, and a
single wire connection going to the sensor. The plate is then screwed
into the grounded car body.

Anyone seen a circuit diagram for this? I'm not 100% sure that when
the door is closed, whether the switch itself is closed or open. My
first guess is that the switch is closed (connected to ground) when
the door is open, the plunger switch is extended via the spring, and
the sensor wire is looking for this connection to ground.

When it wasn't working, it was obvious that the ground connection was
never made since the thing was loose. The headlamp warning chime
never went on (sort of assumed there never was one), and the dome
light (when set between "on" and "off" - some cars are marked "door"
at that setting) didn't go on with the driver's side door. It worked
just fine with any of the other 3 doors.