The reason why car AC is not blowing cold air anymore
(1) Slow refrigerant leak.
(2) Clogged condenser.
Car troubleshooting
A liquid substance is known as "refrigerant" is the lifeblood of an A/C system. Loosely, it's refrigerant which carries the heat from the passenger compartment to the outside of the vehicle, thus making the passenger compartment cooler. However, refrigerant often slowly leaks out of a hose or seal. In time, this results in a steady reduction of the cold air produced by the A/C system.The other possibility in this scenario is that the condenser is clogged. The condenser is the part of the A/C system that's responsible for removing the heat which the refrigerant absorbs from the passenger compartment. It's essentially a mini-radiator that's mounted in front of the regular radiator. In time, the condenser can gradually become clogged with dirt/debris/bugs. This will result in a steady reduction of the cold air produced. The main cause of this problem is likely a refrigerant leak. A clogged condenser (if at all) would only be a contributing factor here.
What to do?
First, check the condenser for dirt/debris/bug buildup and clean if necessary with a soft brush and water. This may improve the problem. If not, you'll need to take this vehicle to a garage and ask the mechanic to "charge" the A/C system (i.e. add refrigerant). Also, ask him to locate and repair the leak. The refrigerant R-12 (also known as "Freon") is extremely expensive. Why? Because, its production was banned beginning January 1, 1996, and therefore all that remains is the Freon produced before this date (i.e. it's a dwindling supply). Therefore, expect to pay a premium to have your A/C charged (often $50.00 - $150.00).