Suburban / Tahoe third brakelight

[Celerity]

I'm working on an issue in another thread, but I came across something else that I thought might deserve a dedicated thread.

 

I've been poking around in the rear heater lately, and then I went away for a bit.  I guess I didn't connect the third brakelight connector. 

 

Anyone know where this is ?  I've been looking, can't seem to find it. I'm sure I did something to knock it free or disconnected.