Buick Lesabre Heater Heater Core

The Heater In My Buick Heats But After A Bit Turns Cold, Is This Heater Core Problems Or Thermostat Problems And If I Get It Flushed Will...

How Do You Repair Or Replace The Heater Core? I Keep Having To Reach Behind The Glove Box To See Is It Is Leaking And It Is.

How Do I Get To The Core To Replace?

If Antifreeze Leaks In My Passenger Floor Board, Does That Mean My Heater Core Is Bad? If So, How Would I Replace It On A 1987 Buick Lesabre...