If have noticed that there are some slighty differences between the MKII and MKIII system 2.
So check your version.
About the "center" socket on your system.
The mkIII has a "hardware" mono signal on both the left and the right pin 3 and 5 at the back.
But the mkII has a somekind of modified mkI chassis where the stereosignal inside the chassis is lead (read: soldered) to the centersocket on the DD/DTS module. With this chassis there will be a monosignal in option 3 or 5 on the centersocket and when option 1 is used, there will be an stereosignal on pin 3 and 5 on the centersocket. (Like with the older avant's option 1 is stereo through the avant's speakers)
So, you could your check your system by looking for a soldered cable at the back of the soundmodule on the centersocket.
So, if you're having an mkII, set your front speakers to "" none" and attach your frontspeaker to the centersocket. Use an splitter for left and right or link the audiosignal from the (i.e.)left speaker to the right speaker.