The horn on a 1996 Chevy Corsica is located just behind the driver's side headlight in the engine compartment. There is also a horn relay switch in the lower part of the dash just to the left of the steering wheel.
Remove the wiring harness from the back of your 1996 Chevy Corsica blower motor. Remove the blower motor retaining bolts. Remove the fan from the front of the blower motor. Reverse the process to install the new blower motor.
I was just browsing online for Chevy Corsica heater core and found they were being sold for $53 at 1aauto.com plus free shipping. It looks like a decent deal to me and the suppliers seem pretty competent in the aftermarket auto parts business. You can check out their website for more details.