The dome light wires can be accessed if you remove the driver's knee panel and then remove the silver panel that holds the side mirror adjustment controls. It is a white 3-conductor ribbon wire with red stripes connected to a 6 conductor plug. You can double-check that you have the correct ribbon wire by tracing the path of this ribbon wire up into the driver's A-pillar. I don't remember the exact wire colors, but looking at the 6 conductor plug from the driver's view point (the back side of the connector):
- the upper left wire is constant ground
- the middle right wire is constant power
- the lower left wire is fade-in/fade-out ground
I tapped into the middle right and lower left wires for my footwell lights. I added a switch on the power wire so I can turn the footwell lights off if I have the doors open for a long time. It turns out that I didn't need it, Scion has a power saver time-out, after about 10 or 15 minutes the dome light power is automatically cut (restored by closing and opening a door). Have fun!