Thanks to all the information that clintg posted, I got the light sensor working on my SR5 today.
My truck already had the light sensor harness, but the 3 wires (red, pink, blue) didn't go all the way to the F9 connector of the BCM. I don't know where they stopped...didn't pull the dash harness apart to find out. Instead I tapped the 3 light sensor wires with PosiTaps and ran 3 new wires to the F9 connector. While that sounds easy, to get to the light sensor harness, the radio and front vent ducts (not for the faint at heart) have to come out.
I ordered the "T" harness that clintg suggested, and harvested 3 unnecessary wires from it as pigtails (with female ends). I soldered those 3 pigtails to my 3 new wires, and inserted them into F9.
I'll have a usable "T" harness left over after I clean up the extra wires, if anyone wants to do this.
This is a rabbit hole...probably 5 hours total.