Just as the title states. I get hot air in front and ambient temp in the rear. BTW, this is on E34

the rear center vents...or the vents under the front seats? If its the rear ctr vents you are referring to...they are doing what they were intended to do...blow fresh and/or "conditioned " air to rear passengers. This is their method for controlling their comfort in the rear.

Heated or warmed air for rear passengers comes thru the floor vents. (I believe this is explained in your owners' manual) ;) Note the color of the arrows in the diagram below:
http://memimage.cardomain.net/member_images/7/web/739000-739999/739005_8_full.jpg

This is one of those reciprocal posts...it comes up every winter...so you're not the only one. :D I suggest turning the vent dial to the "off" position...and if the rear passengers ever feel a bit too warm...they can open the vent and set it to whatever makes them comfortable.