The light itself is OK by now, but we still need a way to switch it off and on. The original Thinklight uses a keyboard combo to do that (fn+page-up), but implementing that would mean hacking the bios or interfacing directly to the keyboard-matrix. I decided to pick another key: capslock. The nice thing with this key is that it's got an indicator led that we can tap, and the key doesn't usually get tapped 2 times in a short period, so we can trigger on that. This means we do need to get access to the capslock-led, which means we need to open up the poor laptop even further...
I needed to unscrew half of the screws in the laptop, but finally the pcb containing the caps-led was found. It's in the middle of the pic.
A close-up. The caps led is the one on the left, and close to it is its switching transistor, which has a point carrying 3.3V when the capslock-led is off.
After a bit of measuring, I found a few key points. I didn't use the 3.3V on the paper btw, there's a 5V-supply underneath L907 (top-left of the PCB). If you have another laptop and want to go hunting for power sources yourself, look at points like that, with multiple via's in parallel, first. You need 5V, btw, cause 3.3V usually isn't enough to light a white led reliably.
Leads connected to the important points and fixed with hot glue, all ready to get connected to the caps-detecting circuit.