If you car is the same as nearly all others that people have posted about, the center vent in the rear only blows fresh or chilled air (with AC on of course). It does not provide heat.
The lever you are referring to should close it for winter time use. If it doesn't then the linkage is disconnected or broken somewhere. Since mine is working properly at the moment, I haven't had the opportunity to tear things apart to know how to fix it...yet