Where do you add freon to a 1999 Chevy S-10?

You don't add freon to a 1999 Chevy S-10 (or any modern vehicle) yourself unless you're a certified technician with the proper equipment. Automotive refrigerant (now R-134a in most vehicles of that era) is under high pressure and requires specialized tools and safety precautions. Improper handling can lead to serious injury or even death.

The low-pressure port, where refrigerant is added, is usually located on the A/C system's low-pressure side. However, finding it and adding refrigerant without the correct equipment is extremely risky.

Instead of attempting this yourself, take your 1999 Chevy S-10 to a qualified mechanic or an auto A/C specialist. They have the necessary tools, knowledge, and safety equipment to diagnose the problem and properly recharge the system. They can also identify leaks, which are a common cause of low refrigerant, and repair them before adding more refrigerant. Adding refrigerant to a leaking system is a waste of money and harms the environment.