Factory 09 non-JBL connector pinouts?

I'm in the process of installing an aftermarket head unit, and I'm a little puzzled about the fourth connector on the stock radio.

Looking at the back of the radio, the far right connector is power and front speakers, the second from right is rear speakers, and I believe the far left is steering wheel controls.

What is the second from left connector? There are only three harness connectors on my adapter harness, so my only guess is that the fourth connector might have the vehicle speed and maybe dimmer inputs that aren't used on aftermarket stereos.

I've been looking, so far unsuccessfully, for a 2009 wiring diagram to solve my mystery...

 

I didn't find the pinouts yet, but I did find a reference to the four connectors and not needing the "extras" here:
Stereo Upgrade BASIC How To... (2nd Gen)