uncontrolable flashers

I have a 91 mercury sable that when ever you step on the brakes or hit the turn signals the flashers go on for approx 30 seconds to 1 min. when you turn on the headlights you cannot see the unwanted flahers going on the outside but you can hear them on the inside (annoying clicking). all of the lights other wise seem to be working. When you turn on the flashers with the flasher button they work as supposed to. does anyone out there have any ideas?