I assume the air is blowing strong out of the defrost?
If, so, the problem is most likely vacuum related along the path to or from the HVAC programmer.
Either a leak or blockage in the supply of vacuum:
1. From collapsed nipples in the connector located at the base of the HVAC programmer (usually this is the problem 95% of the time)
2. From the connections in the programmer itself
3. Along the path or at the connection to a vacuum actuator under the dash
4. Along the supply line from manifold
5 At the researve vacuum tank under the fender- front/passenger side
6. In the line coming from the reserve tank through a wiring harnmess into a Y-fitting before going through the firewall).
Here is the link providing most of the info you need, but again, most times, the connector is the culprit.