The door switches can be perfectly astonishing in that you can have metal-to-metal contact and yet not have continuity. Anyone who has owned a British car is quite familiar with this phenomenon. The culprit is metal oxides that do not conduct electricity. Looks like metal, conducts electricity like a crumpet.

The ground is at the switch as Planetman said; pull the switch and you'll see what's going on. It's just a wire that grounds into the door jamb through an unenclosed NC switch. "Unenclosed" is the problem. That black rubber boot only covers one side, so the switch gets damp and corrodes. Everything is only slightly corroded, only slightly dirty, looks like it should ground properly, yet it doesn't. There's a thin non-conductive layer of oxide.

No problem. Got a can of DeOxIt or battery-post cleaner and an old toothbrush and/or soft wire brush? A little cleaning and deoxidation, refurbish the switch and the little hole that the switch goes into, making sure the switch bites into metal and not rust, and you should be back up and running. Use some of that light bulb gel to prevent oxidation in the future.