1992 Heater/AC control help needed

[waverider]

I have '92 convertible and when I purchased something was wrong with the heater control. The vents won't change, in other words I can't get it to move the air flow to the floor or defrost. The fan speed works, heat lever works, but none of the buttons work. I have replaced the control unit with what was sold as a working unit and the same thing is happening. Does anyone know of any troubleshooting techniques I could use to figure out what is causing this. Or a wiring diagram for the Heat/AC system.It's very had to drive with frost on the widows and my feet are freezing.