Adding accessories

The base harness is the same for the Crown Vic and Marquis, however there are sub harness additions for some options. For instance, they offered low mount lights on some models but not others. The plug for that would be on the main harness but the sub harness came with the optional item.
Being a 2011, you have a rare car. They only made a very limited run of 250 vehicles for 2011 as that was the final year for Mercury, the last Mercury ever was a Marquis that rolled off the line Jan 4, 2011.