How do you fix a 1998 Chrysler town and country door that opens on its own?

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.