Unfortunately, I can't give you a precise location without a diagram specific to that year and model. However, to locate it, start by looking at the compressor (usually located near the front of the engine on the passenger side). Then follow the larger diameter hose from the compressor; it will be the one going into the firewall and eventually into the passenger compartment to the evaporator.
To find it more easily, consider:
* Visual inspection: Look for a larger diameter hose running from the compressor toward the firewall. This hose will likely have a slightly larger diameter than the high-pressure line.
* Touch test (with the AC running): The low-pressure hose will be noticeably cooler than the high-pressure hose. *Caution: be careful as the lines can be under pressure, avoid kinks.*
* Repair manuals: A repair manual specific to your 1999 Chevy Malibu will have diagrams showing the AC system's components and hose routing. You can find these online or at auto parts stores.
Remember safety first. Work on the AC system involves potentially hazardous refrigerants. If you're not comfortable working on your AC system, it's best to take your car to a qualified mechanic.
