Okay, I finally got around to installing the magnetic reed switch to fix my driver door ajar light, and I took a few pictures.
This first picture is for orientation, showing the location under the door panel where I accessed the wire bundle. You can see two red plastic squares which are wire taps. They are just in front of the speaker.
This photo is a close-up of the two wires I tapped into, with a piece of white paper behind them to make it easier to see. I found two green/violet wires, and the first one I tried was the wrong one (of course). You want the green/violet 22AWG wire with the lighter green color. Find them both, and choose the one with a lighter green.
I added a waterproof 2-pin connector between the wire tap (on the door) and the magnetic reed switch (attached to the inside of the door panel). So now if I need to remove the door panel again, I have one extra connector to disconnect. The reed switch is the white rectangle in my palm.
This photo shows where I attached the reed switch on the inside front edge of the door panel. Before replacing the door panel, I routed the wire behind that white batting to keep it in place.
And this photo shows where I taped a coin-shaped magnet to the plastic trim piece on the A-pillar. I originally stuck the magnet on the other side of the trim piece, but for some reason this didn't work. When I closed the door, the door ajar light would go out, correctly. But when I reopened the door, the light would stay off, as if the reed switch was sticking. (In fact, I could get it to open by banging on the door panel right where the switch is attached.) I don't know why, but moving the magnet to the visible side of the plastic solved the problem. I'll leave the magnet taped this way for now, and maybe I'll try later to get it to work behind the plastic. Otherwise I'll glue it in place where it is now taped, and maybe paint it gray.