On 92-99 Bonnevilles equiped with electric climate control, a common part to fail is the a/c programmer. When it does fail, the result is air only blowing out of the defrost vents.
At the factory a grease coating is used when connecting the vacum line connector to the programmer. Over time, this grease causes deterioration of the programmer'* hard nipples
turning them into soft nipples :( which causes a small vacum leak.
VACUM LINE CONNECTOR TO PROGRAMMER CONNECTOR
What I did was remove the programmer'* cover, cut and remove the programmer'* connector, cut and remove the vacum line vonnector, and directly link them, bypassing the failed connection.
After this, I snapped the cover back on, reinstalled into my car, and tested for proper operation.