help with heater

I have a 1992 chev S-10 pickup and have a problem with the heater. The blower works fine, especially on cold air, however, when I switch to hot air, I get very little coming out from the defrost vents on top of the dash.
Most of the hot air (and it is plenty hot) comes out from below the dash from the vent.
Anyone know what the problem is and what is envolved to fix it?
Any help appreciated.
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abc,
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 you.
I did this repair many times when I worked for the dealer. It was an all day project.
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.
GMdude
P.S. I could be wrong. You should look under the dash first, to see if the vacuum actuator has broken away.
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