Sounds like the infamous door ajar pin switch problem so common in the F-150's. The switch is located on the inside of the door right below the internal workings of the latch mechanism. Actually twists 1/4-1/2 turn into the latch mechanism.
People say to spray WD-40 in the latch but over time it will just collect more dirt and gum up.
I had this happen on My old 97 F-150 and I bought a new pin switch and installed in less than a hour. F-250 is probably easier.
I have a 2002 F-250 SD and I was hoping Ford had fixed this problem.
Do what you want, but my advice is to spend the 15-20 bucks for the new pin switch and not have to worry about it for 4-5 years.
Or... You could do the half-***, WD-40, oil slick in your door thing and wait for it to gum up again and watch the light go on and off.
F-250 door pannel is really easy to remove. Sail pannel, Door lock/Power window pannel, and 2 screws and then pull up.