If it is leaking outside the cab by the coolant tank it could be a bad coolant tank cap, a crack in the coolant tank or possibly a loose clamp. If it is leaking inside the cab it is most likely a bad heater core. Check out Flash Off-Road Hummer Accessories, Hummer Maintenance, Modifications and Tech info and read the repair article on heater cores.

 

wouldn't the heater core be the only reason for the very bad smell of antifreeze when the heat is on?

 

Could be a loose clamp on the lines leading into the heater core but usually it is the heater core.

 

a cracked heater core could also drain only out side the vehicles. it leaks the fluid out where the condensation leaks from the a/c drain hose. you would still smell it inside the car but it would leak out side from under the coolant surge tank.