How do you change a heater core on a 1990 suburban with air-conditioning?

The heater core is located under the dash against the firewall on the passenger side. You'll need to remove whatever is necessary to access this area. Once you're back there, you can remove the vent box assembly, which houses the heater core. Replace your core and reassemble the vent box. Reinstall the box under the dash and reassemble whatever else you had to remove to access it. Also, you'll have to remember, before you start this, to disconnect the heater hoses from the core on the engine side of the firewall. Be prepared to catch some cooling fluids leaking out of the hoses when disconnected. BEWARE! Ethylene glycol (anti-freeze) is poison to pets and other small animals. Don't let it drain to the ground without washing (diluting) it away with fresh water. Note: This is not an easy job. I would rate it at about a 4 on a scale of 1-10, most difficult being a 10. Luck!