You need to find the cause, such as a defective door sensor, or electrical short or switch. If you are referring to one of the side sliding doors, as I am guessing you are, these vans have been plagued by these issues for as long as they have been around, and it appears to be even worse as the model years have gotten newer. There are a number of Chrysler/Dodge/Plymouth minivan groups and forums you can find and join that detail this exact problem much better than I can, and usually are a good source for do it yourself repairs.
on a 1998 Chrysler Town and Country how do you reset the On board garage door opener
I have 1998 it is your door jam switchs
replace door latch
From the inside of the door. Its a pain.
The body computer controls the lights, door locks, wipers, part of the theft alarm, keyless entry,
No but there should be a hole in the rear panel a couple of inches from the bottom, you can stick your finger in there and push up on the mechanism to open the door.
Here's a video that I've included below, that shows you how to repair the door lock actuator on the sliding door of a 2001-2007 Chrysler Town and Country.
The door ajar switch is part of the door latch.
The computer that controlls that door needs checked for codes. Then you can diagnose the cause of the code. If there are no codes, then the wiring at the bottom of the door is most likely broken.
You stick the key in the door and turn.
No.
The sensor is part of the door latch. You would have to remove the door pne and unplug the latch.