Where is the heater core in 1990 sierra cutlass?

In a 1990 Oldsmobile Cutlass Sierra, the heater core is located inside the dashboard, typically towards the passenger side. To access it, you'll need to remove the dashboard and possibly some other components, as the heater core is housed within the heating and ventilation unit. It's advisable to consult the vehicle's service manual for detailed instructions on the removal and replacement process.