The cause of the musty smell to begin with is water accumulating in the HVAC box and becoming stagnant. Most likely your AC drain line is clogged and here's a real good thread on drilling a 1/4" whole to fish a wire through the line. It can't be accessed from the outside of your ZJ because the hose drains into the frame. Great design, not!
I don't bother to use silicone on a rubber plug and a piece of duct tape works just fine over the hole with no leaks. I have to clean this line out 2-3 times a years since I live in the woods and the tree crap tends to go down in the grate below the outside of the windshield.
I've actually put a real small hook on the end of the wire and have pulled leaf stems through the hole from the inside.