Due to the difficulty of replacing the air mix door actuator, I'd recommend having the ECC codes read at a dealer before removing the dash. As willwren stated, there are no on-board diagnostics for ECC after 93. For ha-ha'*, check the EXT TEMP display when the air is not warm. Mine read 168* and would only blow hot air if it was set to 90*. On the next setp down (85*) it was cool air. A faulty ambient temp sensor displaying an incorrect temp will not
cause the temp display to flash.
If you remove the inner black part of the glovebox, you can at least see the ait mix door lever and actuator rod. You;ll need a flashlight cause it'* behind a steel dash support and plastic duct work. Set the temp to 60* and you should see the lever (white thing to the left of the red circle in the pic above) move to the left. Change the setting to 90* and you should see it move to the right. the totla travel is close to a 90 degree swing. Make note of any pausing during the travel as this would indicate the gears slipping in the actuator.
Note: the left and right swing may be different on a 96. The idea is to see the full travel of the lever when changing from full hot to full cold or vice versa.
The actuator is about $70 from the dealer and I would not buy a used one. If it comes down to having to replace it, you wouldn't want to go through the dash removal process twice. I should mention that there was a member on www.automotiveforums.com
that supposedly changed the air mix door actuator without removing the dash. He gave instructions on removing the ECM, HVAC programmer and the heater core cover to get at it. Still to this day I have no idea how the connecting rod can be installed and adjusted using this procedure.