Great Writeup! I did this mod a while back. I sourced my light sensor from a 2002 Lexus IS300.
I thought about adapting the IS harness by simply re-pinning the existing 6 wire 4runner connector but there actually is a 4Runner Instrument Panel #2 harness that doesn't require any splicing and is full plug and play. The part number is 82142-35020. This is a 5 wire harness that uses the standard 2 wire connector for the single HVAC sensor and the 3 wire connection to the auto headlight sensor. The harness is cheap at less than $10 and makes the install completely Plug and Play - OEM.
Attached is a picture of this harness with both sensors plugged in.
