If it’s like the 87s I’ve have good news and bad news. The good news is the AC doesn’t have to be touched. The bad news is the dash has to be removed.

The heater box is the white unit sitting over the transmission tunnel. Typical dash removal, hidden screws, and wiring harnesses that need unplugged. The getting the cables loose can be a PIA too. If you go to the trouble to replace the heater core you probably also want to go ahead and replace the water control (temperature control) valve at the same time. A FACTORY service manual is a real time saver for this one.