My door ajar sign is on and the doors are closed

Check the door ajar switch which is located on the pillar post.

It should tell you which one .
Driver Door Ajar Malfunction
Passenger Door Ajar Malfunction
If not a scan tool would be the best way.
Ignition ON, observe the scan tool Driver Door Ajar Sw parameter while opening and closing the driver door. The parameter should change between Door Closed and Door Ajar
? If the parameter does not change, refer to Driver Door Ajar Malfunction.
With the passenger doors closed, observe the scan tool Door Ajar Switch parameter while opening and closing each door. The parameter should change between Door Closed and Door Ajar.
? If the parameter does not change, refer to Passenger Door Ajar Malfunction

Ignition OFF, disconnect the harness connector at the driver door latch.
Test for less than 5 ohms between the ground circuit terminal 8 and ground.
? If greater than the specified range, test the ground circuit for an open/high resistance.
Ignition ON, verify the scan tool Driver Door Ajar Sw parameter is Door Closed.
? If not the specified value, test the signal circuit terminal 6 for a short to ground. If the circuit tests normal, replace the BCM.
Install a 3A fused jumper wire between the signal circuit terminal 6 and the ground circuit terminal 8. Verify the scan tool Driver Door Ajar Sw parameter is Door Ajar.
? If not the specified value, test the signal circuit for a short to voltage or an open/high resistance. If the circuit tests normal, replace the BCM.
If all circuits test normal, test or replace the driver door latch

The door ajar switch inside the door . Door Ajar Switch

Hey Jeff, any U- codes stored in any modules ?
U1900 Missing CAN message(s) Door(s) ajar, PRNDL, 4x4, instrument cluster (IC), running boards, vehicle speed REFER to Section 418-00 . U2023 Invalid CAN message(s) Door(s) ajar, PRNDL, 4x4, instrument cluster (IC), running boards, vehicle speed REFER to Section 418-00 .
L4 CHECK THE VDM FOR DOOR AJAR INPUT

• Key in ON position.
• Monitor VDM door position status PID.
• Close all doors, liftgate, liftgate glass and power running boards.
• While observing the diagnostic tool, open and close one door.
• Does the door status PID change between CLOSED and AJAR states?

When you open an close the driver door you have no PID change ,right ?
L4 CHECK THE VDM FOR DOOR AJAR INPUT

• Key in ON position.
• Monitor VDM door position status PID.
• Close all doors, liftgate, liftgate glass and power running boards.
• While observing the diagnostic tool, open and close one door.
• Does the door status PID change between CLOSED and AJAR states?

The door ajar input is an input to the Vehicle Security Module , then transmitted over the bus ! You'll have to find a diagram for the med. speed CAN data bus .

If the interior lamps and the door ajar light stay on after removing the keys and closing the door, this is usually a sign that the door ajar switch is gummed up.

Liberal spraying of WD-40 in the latch and close open the door a few times. It can take up to 5 cycles of this to get the door ajar latch working correctly again.

There is a thread on F150online.com in the electrical forum ( sticky at the top ) that has pictures of where to spray the WD-40 if you are unsure on this.

door ajar switch in the door latch, they sometime get stuck ,if you try spraying some lube into the latch and open and close the door a couple of time some times they will un-stick otherwise you will need to replace entire latch.

What's your question ? Tailgate ajar light lit on the instrument cluster ?
The Body Control Module (BCM) monitors the lift glass and the cargo door ajar switches. When the switch closes, the path to ground is completed, and the BCM sends a class 2 message to the instrument cluster to display the CARGO DOOR AJAR message.There are two ajar switches for the rear door ,a left an a right ,the third switch is for the lift glass ajar switch . All three of these switches splice pack SP410 At ground point G410

(Shouldn't have any readings when door(s) and/or window closed.) It's the opposite . switches are open with door closed , G410
In the Body Harness, Behind the RR quarter trim panel near the D-Pillar
It turns out it has nothing to do with rear door. Something about drivers door actuator. NO IT DOESN"T !!!!

Find out which door is showing ajar and replace the latch or door ajar switch.

Normal Operation When any door or the liftgate is opened, the ajar switch provides a ground signal to the smart junction box (SJB). When the door or liftgate is closed, the ajar switch opens, removing the ground signal. The SJB uses the multiple inputs from the ajar switches to determine whether the doors or the liftgate are in the open or closed position. The SJB then provides a message to the instrument cluster over the communication network. The message center uses this information to display the door/liftgate ajar message.

I suspect your door switch is not closing all the way.

try spraying some rust penetrant or WD40 into the door latch and open and close the door several times or more and see if the ajar light goes out and the belt is operating ok