The heater core in a 1993 Chevy van (assuming you mean a full-size van like a G-series) is located
inside the dashboard, behind the instrument panel. It's not easily accessible and requires significant disassembly of the dashboard to reach it. You'll likely need to remove parts of the dash, possibly the glove box, and potentially the steering column to gain access.
Because of its location and the complexity of accessing it, heater core replacement is generally considered a major repair job, often best left to a professional mechanic.
