Door Ajar
 

Is your door adjar light on with all your doors securely shut. Well you need to clean the door latches on the doors. I use compressed air then follow with brake cleaner. This will clean the connections that create the ground that keeps that dummy light off. This usually comes up about every 3-6 months depending on how dusty of an area you live in.

Door ajar part 2.
 

I do that just like BC says. But if your superduty has had years of neglect. It may not be enough. The dirt and grime in the door can actually get into the acuator and gum it up not allowing the plunger to come back out at all. On top of that the plunger can just plain wear out. So if it is totally shot get ya a new one and make sure to follow BC's suggestion on cleaning it everyonce in awhile.

One thing you don't want to do is use a lubricant like wd-40 all that does is gather more dirt and dust faster. Use some type of cleaner that evaporates like brake cleaner.