your heater core is probably made of plastic and has finally cracked like they all do. See the dealer or a professional like myself.
If it's leaking from INSIDE the firewall (away from the engine compartment) you probably have a leaking heater control valve or damaged heater core.
If its inside the cab at the firewall, its probably your heater core which is located on the passenger side under the dash. This repair is usually not much fun. If its leaking in the engine compartment near the firewall, its probably a heater hose that is failing. This is an easy repair
The radiator will need to be replaced. Sometimes, it may seem that the radiator itself is leaking but it could be the radiator "hose" that is leaking. In this case, the radiator hose needs to be replaced and clamped tightly.
Loose hose connectors, damaged hoses, bad head or intake manifold gasket, punctured radiator or reservoir... all depends on where it's leaking at.
Check radiator hose, leaking water pump, look for hole in radiator, leaking coolant bottle or leaking seals.
Find the source of the leak. Most likely areas are a brake line at the wheels, or the master cylinder at the upper part of the firewall in the engine compartment.
If your home radiator is leaking, it needs immediate repair.
Maybe it's not the radiator. Maybe it is one of the radiator hoses that is leaking. Or possibly the radiator cap or a level sensor.
heater hose?
You need to explain more on where exactly the coolant is leaking. There are several possible reasons why your coolant is leaking. If it's from your main hoses that lead to and from the engine or radiator, you need to check the condition the hoses are in. If they're in a decent condition, it's not that. If it's leaking around where the hoses are connecting, it's probably not tight enough, or there is possibly a clog in your coolant line. Also get your radiator checked. If it's leaking into the passenger compartment, it's most likely the heater core.
"transmission fluid leaking from fitting below radiator cap after replacing radiator on 99 surburban
Leaking radiator? Leaking or loose radiator hose?