Dash instrument lights will usually not come on when I turn the switch on

I have a 2001 Ford Taurus. The dash instrument lights will usually not come on when I turn the switch on. Instead the hazard dash lights come on and stay on - they don't blink on and off; the dash is completely dark. Usually after 15-20 minutes of driving like this the dash lights will suddenly come on like normal and the hazard lights will go out. What is causing this?