Harness behind head unit, NOT h/u harness

Anyone know what the harness/connectors directly behind the head unit is? I had my factory head unit out and found this harness disconnected.  Large plugs, about 2.5 inches wide. I plugged it back together and haven't noticed any change on anything, good or bad. 2003 Z66.

http://www.chevyavalanchefanclub.com/cafcna/index.php/topic,44768.msg707821.html#msg707821

The harness in question does not plug into the radio at all. It's just two big connectors behind the h/u that plug together. Electronics are not anything resembling a strong suit for me. Most of the time wiring schematics are Greek. Just trying to figure out what it was. Had the head unit out to remove the cd mech from the radio chassis.