Paul,
After all these years, some of the knobs have developed a strong attachment to their position. I have had success with a penetrating agent with a flexible straw-like spray attachment. PB Blaster,or a silicone spray. Be sure to depress the bullet that is usually on the underside with a not too pointed object before trying to remove the knob. I must admit to using a prybar behind one of my knobs. The risk there is ruining the part it is attached to, which if it were a wiper switch would be a bad deal. Good luck.
-Dennis