1994 Chevy Lumina Dash Lights Stay On

The car has always had the dash illumination lights come on for about 30 sec when the doors are first closed then go off. Like they were on a time delay. Today I noticed that they were stuck on. When they are on they cannot be controlled by the dimmer, the only time they go off is when the dashlight dimmer is moved to turn on the interior lights, then the dash lights will go off. The lamps will come back on when the interior lamps are turned off. When the tailights or headlights are on the dash lights can be dimmed by the dimmer and seem to function normally until the ignition is turned off then they stay on.