What is the difference between a front and rear heater core 1999 Chevy Suburban?

There's no such thing as a front and rear heater core in a 1999 Chevy Suburban (or most vehicles). Suburbans of that era have a single heater core located within the dashboard, supplying heat to the entire cabin. The heat is distributed to the front and rear vents via a system of ducts and air blending doors. While the rear vents might be controlled separately, they're fed by the *same* heater core.