1999 F-150. New wiper motor 2 years ago. Wipers will randomly cycle 1 or 2 times about twice a month, throughout the year. Weather is not a factor. Switch is in the off position. Why?? and how do we fix it?
Probably residue in the switch. As electronics and switches get smaller and smaller there is less space between contact surfaces. The grease the factory puts there when they built it may have accumulated dust ( over ten years )and occasionaly connects a circuit.
Sometimes the wire ends in the harness plug come loose and make for an electrical malfunction puzzle.
This switch comes off the left side of the column pretty easy. take the column trim halves apart and two screws hold the switch to the side. wiggle the wires and test pull each one, none should come out individualy. If one does you’ll need the grey plug block E6DZ14489M. If the switch…YL3Z13K359ABA. Also look for chaffed wires. Someone who tilts thier column a lot will find loose wires in the plug or chaffed wires somewhere.
I don’t know how the Ford F-150 is wired, but if the wipers are connected as they are in my 1978 Oldsmobile Cutlass, the switch completes the circuit to the chassis of the car. The positive feed is always hot. If this is the way your truck is wired, there could be a wire hitting a metal part of the body before the switch which is completing the circuit.
BTW, I puzzled over why my car was wired this way. I finally reasoned that the manufacturer could save about a foot and a half of wire per vehicle.
The wiring is much different from the 1978 example that you give. There is a multi-function switch that has three or four leads to the electronic control module that controls the wipers. The switch puts various resistances to the control module on each wire to indicate what position it is in. I think Ken is right that it is most likely the multi-function switch or the wiring at it. There is a lesser possibility that it is the electronic module or a switch contact in the transmission of the wiper motor assembly. I don’t know if the 99 has a control module separate from the wiper motor assembly (like they used to do), or if it is a combined unit (like they are doing now).