Remove the glove box liner and look for a vertical manifold at the lower right of the glove box. Use an auxilliary vacuum source and remove each line from the manifold one at a time and see if they all will hold without leaking. These are the lines to the vacuum pods under the dash.

All the time you're doing this, cross your fingers and hope that they all hold. If one is leaking you have a bad pod under the dash. To repair it, you will have to remove the dash. Don't even dream about replacing only the leaking pod, replace them ALL because it is a 15 hour job.

The other thing to check is to make sure that there is no coolant flow to the heater. With the engine warm and all a/c controls on max feel of the line into and the line out of the heater control valve near the battery. It should be hot on the input side and not nearly as hot on the output. If the output is hot, then you need to start checking the climate control and it's signal to the heater control valve.

Hope this helps,