Yet another dome light issue

[USSkoval]

I have a problem with my dome light that seems to be different from every other (I searched a bit). It's a '79 crew cab with the center dome light and one under dash courtesy light. At first, neither would come on with the door open or the headlight knob rotated, and both front door jamb switches are seized. I replaced both bulbs, and now the courtesy light will come on with the headlight knob, but not the dome light.

Now here's where it gets weird (to me at least). With my volt meter, the dome light has a constant 12 volts on BOTH wires. So I got curios and tested the working courtesy light. It's the same way, constant power on both terminals. How is this possible, and why doesn't the overhead dome light work?