Defroster won't turn off

I have a 94 Eagle Vision. Occasionally, the defroster will turn on by itself with no digital readouts. There are two ways to turn if off - remove the fuse or slam the side of the console with a rubber mallet. When it turns off, the heating/cooling system will then work properly. Any thoughts?

There’s a loose connection somewhere.
You can try flexing the control wires until it happens to locate the bad connection. It should be pretty cheap and simple to fix.

Thanks for the info. Something must be loose, but that console is a bear to remove or work behind. Interestingly, it would seem that the problem only happens when the fan control is turned all the way up. If you turn the fan to its lowest position and hit the off button, the defroster turns off and everything works fine. I told my wife to just use the system in auto mode.