Had the same thing happen on my wife's 97 Mountaineer years ago. After hearing her complain about the radio for weeks, I decided to take out the radio. I was in process of taking out the radio itself from the dash when I thought about the problem. The only logical thing that could make it continuously scan (other than a bad radio) was a stuck scan button but not only on the dash radio face plate . My wife the pack rat had somehow wedged a book between the rear radio control panel and the back seat. The scan button was being physically pushed in by the book. Check both the scan button on the front of the radio and the scan button on the rear radio control panel. I bet you the rear control panel button is stuck somehow.
