Water is getting into the cockpit, are there any drain holes in the bulkhead that should be cleaned?

 

First place to check and one that is often overlooked is the drains in the doors and rocker panels. Make sure those are clear of dirt and debris, then move on.....

 

could be the air con water outlet pipe at the front passenger side is displaced.

 

I just recently got myself a 1998 B2500 and experienced the same thing!

In my experience, it's the cabins outside rear break light!
After a good number of years the gasket deteriorates and your usual car wash or rain will seep against the cabin wall without one seeing it.

One phillip screwdriver and it comes out.
Some RTV silicone will fix the issue but if you could find the actual gasket or some similar material substitute it would be better.One day you're going to have to replace those bulbs.