a/c only blowing through defroster, somtimes through vents

[jsinitie]

Hello, Most of the time, the A/C in our 07 x5 3.0 is blowing from the defroster. Occasionally it will blow through the main vents on start up. It never changes after start up. We recently changed the battery and for a little while it corrected the problem, however it is occuring again.
When it is coming out of the defroster, the rear ac does not work either, although it works fine when air is blowing through the main vents.

Any thoughts? It sure is getting hot in Texas.
Thanks,