I replaced my heater core on my 99 Blazer a year or so ago. I know that I had to do most of what was said in the older post, so I started reading back through the posts in this thread. I believe you may find that the 98 core is different than the 99, 00, 01, etc.... Anyone attempting this should concider what Bob is saying, but you may find on the new vehicles you have your work cut out for you. It took me 7 hours. 2 of which where spent fighting getting the core to line back up...until I realized I needed to cut another tab under the dash to loosen things up enough to fit the tubes through the firewall (I felt like an idiot for messing with it for that long before I looked). This job isn't difficult, but it will test your nerves.