I recently purchased an 06 F150 FX4 and when I bought it the reverse sensors worked as they should. But as of a few days ago, the interior light shows that the sensor is "off". Obviously i have pressed the button to turn it on but it refuses to change the light. Occasionally it will be "on" when I start the truck but as soon as I put it into reverse it almost immediately turns to "off" again. Anybody have any ides on how to trouble shoot this problem?