92 Corsica

It's in the center of the heater/ac case behind the dash I posted the instructions on how to change it.
REMOVE OR DISCONNECT

1. Disable the Supplemental Inflatable Restraint (SIR) system, refer to Disabling the Supplemental Inflatable Restraint (SIR) System. See: Body and Frame Interior Moulding / Trim Air Bag(s) Arming and Disarming Service and Repair
2. Negative battery cable.
3. Drain cooling system.
4. Raise vehicle.
5. Drain tube from heater case.
6. Heater hoses from heater core.
7. Lower vehicle.
8. Console (if equipped).
9. Right and left sound insulators.
10. Steering column opening filler.
11. Floor air outlet duct.
12. Heater core cover.
13. Heater core mounting clamps and remove heater core.

Under the center of the dash, there is a tray to remove that will expose the core.