in response to a message on the Corvette Forum # 311874
.? the air conditioning, defrost .. all come out of only the heater vents

Most likely a vacuum or actuator problem. Check out the color Climate Control illustrations for troubleshooting vacuum problems. Actuators are shown in a slow loading Adobe .pdf file.

It does not sound like you need to replace that entire expensive climate control mechanism. If the levers and linkage on that climate control mechanism are OK, you would probably only need to replace the rotary switch which shunts the vacuum to different actuators powering the climate control doors: i.e. heat, cool, defrost. Yours should be the plastic type, while earlier 69-76 use the zinc metal ones. If you are not sure of the style check out my illustrations and count the number of nipples. Some say they have repaired these rotary switches using Vaseline, however I'm not sure I would wish to remove and replace everything to re-lubricate this switch.

However, even your rotary switch may not be bad. Perhaps you simply lost the vacuum source - it's a small plastic hose (if yours is original) and enters a fragile t-pipe connector often located just to the inboard side of power brake booster on the firewall. Often that tiny nipple is broken off and you loose vacuum control for the climate settings.