I just went thru this on my wifes Olds 98. Hers has the same pass. control as yours does. It sounds like the airmix acuator alright. I got my replacement off an Olds 88 so hopefully yours will be in the same place.
Pop open your glovebox, empty it if you need to, move the rubber stops at the rear of the glovebox so that you can fully open it.
Using a flashlight look towards the firewall directly in back of where the top of the glovebox would be and you'll see the module in picture 12 (but from a side-on view) from this http://www.trialsnuts.com/ecc.pdf
thats the airmix actuator. turn your car on and you should see the white and metal control arms move towards the left and then back to the right. The movement should be smooth with no stutters or stops. If not then you need to replace it. The metal rod should be connected to another white grommet on the left / threaded end that isn't pictured .
I think your rod either popped off or your actuator is really really bad. Most of use get full cold when the gear cracks (pic 16) and no heat.
To replace it here are the procedures I wrote up for my wifes car.
1: empty the glovebove
2: Remove glovebox by removing the 4 screws securing glovebox to the dash frame at the bottom of the glovebox. They are either 5.5mm or Phillips head screws.
3: Remove the module above the airmix actuator by removing the one 6mm screw, slide it towards you and position out of the way to the right. Theres no need to disconnect the wires.
(This will give you enough room to get a small 1/4" ratchet onto the Airmix Actuator screws without much problems.)
4: pop the airlix lever off at the threaded end of the rod.
5: remove the two 5.5mm screws from the airmix actuator. The second one is on the far side of the actuator almost directly across from the one you can see. Feel along the rear side until you find the circular recessed area; the screw will be in there.
6: Disconnect the electrical connector.
Installation is the opposite of removal. Be sure to calibrate before connecting the threaded rod.