* Condensation: Air conditioning systems remove moisture from the air. This moisture condenses on the evaporator coil and needs to drain out. The drain is usually a small tube or hose.
* Clogged Drain: The most common cause is a clogged drain line. Leaves, debris, or even mold can block the drain, causing water to back up. Since the AC pressure pushes the water, it might overflow and find its way into the passenger compartment, particularly if the drain is near or above the passenger side.
* Drain Line Disconnection: The drain line itself might be disconnected or cracked. This is less common than a clog but still a possibility.
* Evaporator Case Leak (Less Likely): While less probable, a leak in the evaporator case itself *could* also allow water to seep into the cab, but usually this would leak regardless of AC operation. It's more likely to leak if the AC is running because the system pressure is higher.
* Other Leaks (Unlikely): Leaks from the windshield or sunroof are usually independent of the AC system and would leak regardless of whether the AC is on.
In short: Start by investigating the AC drain line. You'll likely find it near the firewall on the passenger side, usually a small rubber tube. Check for clogs and ensure it's properly connected. If you are uncomfortable working on your vehicle you should take it to a mechanic. A mechanic can also check for less common issues if the drain line is not the problem.
