Something simple you can try, and you probably already tried this but.......
I just got used 05 burb. Was looking throught the owners manual and thought I saw in there that if you have the temp controls off, then it will default to blow air at the temp it was set on before you turned them off. Mine was blowing cold in the rear also. Had my son turn the rear unit on and look at the temp, was set at about 70 degrees, so we turned the temp up and then it blew warm air wether or not the controls were actually "turned on" or not back there.
Have not ridden back there myself yet to really check it - so we are going on the word of a 12 yr old