2006 Ford F-150 REAR OBJECT SENSOR MALFUNCTION

The button on the lower dash that turns on/off the rear bumber object sensors is "stuck" or lit in the off selection. Pressing the button multiple times will not cure the problem. Cleaned lenses on the bumper sensors, no help. All wires seem to be connected as needed as best I can see. Oddly, some days, when the temperature outside is a little warmer, it works. This problem first began when we had our first real cold snap this winter. Do I have a bad sensor(s), switch, or both? How do I find out?