Where is the heater core on a 1988 Chevy G20 van w air and v-8 motor located?

The heater core in a 1988 Chevy G20 van with air conditioning and a V8 engine is located behind the dashboard, inside the firewall. It's not easily accessible; you'll have to partially disassemble the dashboard to reach it. There's no single "easy" access point – it varies slightly depending on the exact trim level and dashboard design. You'll likely need to remove parts of the dash, glove box, and possibly even the steering column shroud to get to it.