The heater core is leaking into the AC drip pan. That hose is the drain to let water run onto the ground. This is real common on GM trucks, especially when the coolant isn't changed regularly. Repeat failures are common due to the coolant becoming acidic.
Grab a digital voltmeter and ground one probe, and touch the other one into the coolant in the radiator. If you see a high voltage reading, flush and refill the cooling system along with replacing the heater core.
Thursday, August 4th, 2011 AT 7:18 PM