This is the odd thing... the rear vent does not blow out heated air, only air conditioned or unheated outside air. However there are vents underneath the driver and front passenger seat that blow air to the back in footwell mode even in the 08-10s with the rear centre A/C vents, so if you set the automatic climate control to the temp you want and it's in heating mode, it'll automatically go to windshield and footwell mode meaning air for the rear passengers will come out from under the seats.
If you find it's not warming up quick enough back there, go to manual mode, set the temp up high and blast the fan speed up high until it's warm enough then go back into automatic mode, as I find that you have to blast the fan speed to 5, 6, or 7 to feel any air come out from under there. Heat rises though so the placement really is not much of an issue. If worse comes to worse, you can get one of those 12V Plugin Heaters I've seen, but for me the rear of my 5 has never heated up any slower than the front of the 5. It just takes awhile for the engine to warm up enough to provide heat.