I had a 98 and the dealer told me after my 850. 00 core behind the airbag and about 7 hrs R&R in the dash that if you park it outside and if it could be under trees that the oak leaves or pine needles contain tannic acid and the liquid or leaves accumulate in the bottom of the AC/Heat intake plenum from the outside grill intake that they can cause corrosion of the core and corrode a pinhole in the core , i said great here is my check , i thought, is the core that thin ? guess so cause it was up in arkansas in the summer and about 110 degrees after work in the shade and 100% humidity , pretty much sucks as a design flaw due to no cabin air prefilter or air box cleanout .
that or o-rings or hoses or all of the above oh yea then the ac ducts or flappers would open or close randomly under heavy acceleration and the defroster would come on in the summer it was a check valve to close to the manifold and the heat was messin with it replaced it and moved it farther away from the heat source and it lasted for a while then would happen again,
Austin Diesel