Door ajar stays on usually when it rains - 1999 Ford Expedition

Usually a problem with the sensor in one of the door latches.

It can be issue with faulty sensor or a wiring issue.Troubleshooting is required.
Click the help link shown below:---
Door Ajar Light and Dome Light Remains ON? http://whoisbyme.blogspot.in/2012/06/2003-mercury-sable-door-ajar-light-and.html
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Door ajar light stays on? http://technoanswers.blogspot.com/2012/01/door-ajar-light-stays-on.html

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Thanks.

You will need to spray each door latch including the rear hatch liberally with WD 40 or spray lubricant on the latch and open and close each door several times after spraying. This will usually be sufficient to get the door sensors to detect when you have closed the doors. If this doesn't do it, it will hep to figure out which door is intermittently causing the door ajar light to stay on.

one of your door latch assembly is showing the door ajar,you will need to take it to Ford when it is raining and light is on for them to tell you which assembly is at fault

dodgy door switch .

the problem is door ajar switch located on one of the door latches just inside the door. what usually works is spraying wd40 or an electronic lubricant prayed liberally into the door latch and then opening and closing the door several times. the stuck switch is usually on the door that is the least used door like the passenger side rear. let us know how you make out.

one of your door switches is out of adjustment or defective.  You can test this by pushing in the switches manually.  

door ajar switch is usually the problem switch is a little button on the door jam that opens the circuit to the light when the door closes

you may just need to adjust your door jams they may not be closing properly, the other thing you can try is opening one door at a time and putting your finger on the switch usually in the lower l/h corner of b piller and see if the light goes off if not this may be the problem and replace the switch

I know the problem. Mine does it too. The fix is easy. Open & close doors to find which one is causing the problem. Open that door panel and find the release latch at back of door. There are 2 wire harnesses attached to the latch. Unhook the wires and spray sockets of latch. There is a ballbearing that gets stuck in switch. Lubricate that ballbearing and work latch manually several times. This problem will reoccur occasionally especially when there has been a lot of rain. Good Luck