In my 1999, it is the driver door. You can figure out which door it is by sitting in the van with engine on, all doors closed (door open light lit, of course). Open each door one at a time and listen for the ding. For the doors the van knows are closed, you'll hear a ding. Close each door after opening and listening for the ding.
If it is one of the side doors, then you can clean it as described above. If it is one of the front doors, you are in my boat. Ford engineers got fance with this van and put the door status switch inside the door where it monitors the door latch to determine door open/closed. This was a terribe idea. My understanding is the door switch gets dirty and doesn't pop out when the door opens, so the van always thinks your door is open, even though it isn't.
You can clean the switch, but I have more things to do than take my door apart, I put tape over the light, problem solved.
bN