A few days ago my 2006 F 150 showed an intermittent ABS light. The light would come on when I started the truck but would eventually go out. Then it started to stay on constantly, unless I shut the engine off and then immediately turned it back on and then the ABS light wouldn't come on. The truck's brakes operate normally. This has been a good truck that has never broken down or required anything other than normal maintenance in over 105,000 miles of use.

 

If you haven't already seen these they may be helpful:
https://www.vehiclehistory.com/questions/why-does-the-ford-f150-abs-light-come-on-130992

https://www.ford-trucks.com/how-tos/a/ford-f150-f250-why-is-my-abs-light-on-356396

Here's a wiring diagram of the ABS system from a 2004 Ford manual

 

Before looking at wiring schematics, you need to get the ABS code read to see where to start diagnosing.

 

Took the truck to Auto Zone and used their free plug in tool. According to the tool, it has a ABS fault or a wiring continuity fault in the rear sensor. When it first started to fault, the ABS light would go out if I turned off the ignition then restarted it. Now the ABS light stays on all the time and will not go out with a restart. The truck has never been in an accident or has it had to traverse deep water, and that leads me to think it's the sensor. I will also check all the connections under the truck to insure there is no corrosion on any of the terminals.

 

My 2004 5.4 Lariat had this same problem in the past, got code C1230-E0 (Wheel Sensor Rear Center Input Circuit Failure) replaced rear sensor, and that fixed it.