With some colored wiring charts from the Yahoo Group, and then testing on the new S90 harness, things finally started to click. The trick is to understand the basic theory underneath everything, at least enough that the color of the wires doesn't even matter (because they'll never be the same as what's in your diagram). In other words, it's good to know that there are usually three wires going up from the main harness to the headlight: a wire that's hot when ignition is on, a wire that's hot only when the lights are on, and a ground wire for the neutral switch.
The blue/red wire on Astrid goes to the battery positive, and then I just grounded the negative to the frame. Then I connected the blue/red wire to the red wire at the key switch, which would simulate the ignition being turned on. Then I also connected the brown wire to the red wire, simulating the lights being turned on.
And wouldn't you know, as soon as I opened up the headlight case, I found the neutral light gleaming!! What a beautiful sight. With the multimeter I tested the headlight wiring and found that the blue wire was powering the low beams (switch in the middle position) and the white wire powered the high beams (switch at the top position). That's actually backwards according to the diagram, and I think the diagram is right, since the blue wire had a splitter on it to feed the high beam indicator on the dash - and that wouldn't exist for the low beam. When I tested the bulb (stolen back from Viola), everything worked there, too.
Next I tested the contact breaker. With the multimeter I saw that when the contacts were open there was voltage, and when they closed the voltage dropped (opposite to what you would see using a static timing light).
Finally, I thought why the hell not and just tested for spark.
