Under the dash pass side most likely. yo find exact spot trace heater hoses to firewall under your hood. You have 2 of them.
It is in the heater box under the dash and it usually means removing the whole dashboard assembly.
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Drain the cooling system. Remove the water hoses from the heater core. Remove the heater core retaining bolts. Reverse the process to install the new heater core.
The heater core is inside the hvac housing behind the dash.
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To change the heater core in a 1986 Toyota Supra, first disconnect the battery and drain the coolant. Remove the dashboard and climate control assembly to access the heater core housing. Disconnect the heater core hoses from the firewall, then unbolt and remove the heater core itself. Install the new heater core, reassemble the housing, dashboard, and reconnect the hoses, ensuring to refill the coolant before testing the system.
The heater core is inside the hvac housing under the dash.