99 Monte Carlo 3.1 engine. On around 2002 the ck engine light went on Auto Zone said it was the cap so I replaced the cap from a dealer. A Few years later the light came on and auto Zone said it was a switch on the evap system. I ck the switch on the canister up front and it was fine but the one in the rear wheel well I could not get out to ck it. The old tank started to leak and I thought maybe the tank was crushing from vacuum. I removed the tank and put a new on in and it is doing the same thing. Is it the cap the switch or something else?

 

sounds like the vent is stuck closed. Im not sure exactly where it is on your car but on the 6th gens its behind the drivers rear tire, near the filler neck. Remove it and try to blow into it, if you can then its good but if you cant its no good. With no power to it, it should always be open.