the most common leak complaint on any A/C system is from the evaporator inside the HVAC box under the pass side of the dash. the reason for their common falure is pretty simple...
When the a/c system is on, the cold surface of the evaporator attracts moisture like a cold glass of soda... that moisture builds up and drips out of the box and thats why yo get that water puddle under your car when its running. What can happen is the hole becomes blocked, and the water sits at the bottom of the box. if any leaves or organic material get in there they decompose in the water and raise the acidity of the liquid.. this in short, rots the evaporator sitting in the water.
sometimes if the evaporator is leaking good enough, you can see the tracer (if it has been added to the system) with a black light coming from the hole in the HVAC box... i cant remember off hand where ours are at, but a good undercar inspection will get you started... and remember... A/C work MUST be done in a shop by a certified technician