You have a short. It's not really a Ford van problem. Lots of older cars develop gremlins like this.
What year is your van?
Does it have keyless entry (RKE)?
Is it a fully built out Sportsmobile?
There is an RKE control box on the older vans behind the B-pillar. I'm not sure about vans newer than 2005 or so but they probably never changed it. It's on the wall behind your seat, driver's side, under the window if you have a passenger van. If you have a '50' floor plan or one with a galley or cabinets down the driver's side you're in for a real party to get at it.
This is worst case though. Any of your door lock switches on any door could've gotten wet. Do you have that low mounted door lock button on the rear passenger barn door? Sometimes SMB and other converters bypassed and hid it. If you remove the trim panel on that door you might find it hidden in there. This is second worst case scenario.
The best scenario is this: One of your front door switches got wet. It's that or Satan.