STIS service manual electrical diagrams show connector N20 as "OPTION CONNECTOR" on the No. 1 Roof Wire.
It shows a five pin connector near the rear view mirror. It's a white connector, Toyota part number 90980-12365. That five pin connector is used on other models as part of wiring in the seats, so a bit more work would probably expose the mating connector part number if someone wanted to roll-their-own cable to plug right in.
It's wired with a Ground and two 12V lines.
One 12V line is on the 10A ECU-B No. 2 fuse (constant battery power).
The second is on the 10A ECU-IGR No. 2 fuse (switched "ignition").
