My '81 didn't come with air but my grandparents had it put in shortly after it was new. Now, when you turn the fan on the a/c automatically come on. When you try to select defrost or heat, the air remains on. Does this mean my a/c-heat control is bad or is something else wrong? I have no clue. Can you guys help me out? Thanks

Have you looked under the dash to see if the vac hose to the air motor is still connected. Is the vacuum hose going to the a/c unit inside and under the dash ok? It may be broken under the hood, or cracked at the manifold connector. Give it the once over first.

Also, the a/c-heat/defrost selector is really hard to move. It is very stiff and will stop in selective points that arent under the a/c-heat-def. letters. Also, when you move the lower hot/cold selector, the air remains cols. I forgot to add this earlier. Hope this helps. Thanks