If you're looking to recharge the air conditioning system, the access points for adding refrigerant are typically located on the low-pressure side (usually a service port with a schrader valve) and sometimes a high-pressure side port as well. These are usually found on the air conditioning lines near the compressor or condenser. They will be capped. You'll need to consult a repair manual specific to your 1996 Monte Carlo to locate these ports precisely and safely.
Warning: Working with automotive AC systems involves refrigerants that are harmful if mishandled. Improperly charging the system can damage the compressor. It's best to have a qualified mechanic perform this task.