Coolant leak

You know we can't see it over a computer, but the most common cause on GM vehicles is a leaking heater core, especially if you still have the miserable red Dex-Cool antifreeze. If the heater core is leaking, you'll see coolant dripping from the AC condensate drain hose on the passenger side of the firewall, under the hood. That's a 4" long rubber hose hanging down, with a 90 degree bend in it.