The core is located in the middle of the dash, toward the bottom of the floor.

1- disconnect the battery , drain the cooling system.

2- disconnect the heater hoses from under the hood.
-Plug the end of the heater core pipes(so you dont get spills).
If the hoses are stuck try twisting them with pliers, or cut them and replace.

3- Remove the right lower sound insulator panel(below the glove box) and the lower Left kick panel, and the one below the steering column.

4- Remove the console and glove box.

5 - Disconnect the drain tube from the heater core.

6- Remove the floor air duct from below the heater case.
A - Malibu and Cutlass(only) they need the AC refrigerant dispelled by a dealer or other qualified shop, dont disconnect the ac lines unless this was done!)
B- These models also will need the instrument panel removed from the passenger area.

7- Remove the heater core cover (http://i28.photobucket.com/albums/c242/xeroinfinity/auto%20odds-ends/corehousing.jpg), 9 screws.

8- Remove the heater core clamp (http://i28.photobucket.com/albums/c242/xeroinfinity/auto%20odds-ends/heatercore.jpg)screw and clamp, then slide out the heater core.

-- Reinstall in reverse order.

* Make sure all the insulation and sealing materials are put back in place around the heater core.

Good Luck

I'll get some more pics up soon.