anyone know why water leaks into the cab from under the dashboard?

Do you have a leak when the windshield wiper "tray" fills up with water or all the time ? Look to see if the water is building up in the wiper mechanism "tray" (drains not cleaned out) and causing it to overflow down the inside of the firewall and enter into the cab where the wiring harness, steering column or heater/AC plenum box passes thru the firewall.

Seems to be after I've used the ac for awhile. And the condensation container is full.

Check to make sure the condensation drain line is connected properly or even installed correctly. Condensation should not build up anywhere, it should drain out immediately to the ground.