Yes, I know what the problem is. The vacuum actuator for the "mode", has
broken away from the heater box. You could try to fix it with epoxy, but
this seldom works.
The problem is, that the vacuum actuator is attached to the defroster
duct. In order to replace the defroster duct, you need to remove the
dashboard. I mean the whole thing, until you see the firewall in front of
I did this repair many times when I worked for the dealer. It was an all
The actuator is located above the gas peddle. You will need to remove
the black cover under the dash, on the driver's side to inspect it. Then get
on your back, under the dash, with a flash light.
Maybe someone in this group has a better fix? I don't, at this point. I
left GM in 97'. There was no quick-fix posted then.
P.S. I could be wrong. You should look under the dash first, to see if the
vacuum actuator has broken away.