RE: changing heater core
For a 96 you'll have to remove the entire dash. Remove all the kick panels, the stereo, hvac control panel, drop the steering column, remove the 4 bolts in the front of the dash and rock it back to access the wiring harnesses, unplug harnesses, remove dash from vehicle. Once the dash is out, you'll see the heater core bolted to the firewall, behind a plastic cover. You'll need a 5.5mm socket to remove that plastic cover. Once the cover is off, it's just 2 big spring clip brackets holding it in place. Swap it out and put everything back together again. I think it took me somewhere in the neighborhood of 3-4 hours to do this on my 96 Jimmy. It's not hard, just alot of stuff to remove. In terms of tools, all you'll really need is a 1/4" drive socket set and a 5.5mm socket.
That's not step-for-step, that's just the gist of it to let you know what you're getting into.
1996 Jimmy 2-door 4x4 (daily driver)
1994 Camaro Z28 6-speed (autox toy)
2005 Kawasaki 500R Ninja (gas saver)