From your description, we don't know if there is nothing happening with your rear air, or if it runs but the air isn't cooled. Does the front air work?
The last time my Burb's A/C stopped working was because the pressure switch had failed. One minute I had air and then nothing.
Instead of guessing, get a repair manual and go to the section on A/C trouble shooting. It will give you a step-by-step list of things to check based upon your problem.
Good luck.