I had the same problem. You can buy harmless dye at an auto shop to put in so you can trace the leak's source.
Intake manifold gasket
It leaks from the failed part.
Check coolant leaks with a visual inspection. Clean the engine, then see where the coolant reappears. Look under the water pump. Check engine oil to see if coolant is leaking into the oil.
under the coolant reservoir
under the altneter
Sounds like you have a loose or damaged hose to your heater or perhaps the heater radiator is damaged.
A coolant additive that seals some external leaks and closes off internal coolant passageways including stopping flow in a radiator or heater core. There are different products for different applications. There is a radiator leak sealer which uses "organic material" to seal leaks, and a head gasket/block sealer that uses sodium silicate to solidify under high temperature.
Under the plastic engine cover midway along the rail carrying coolant - opened with Allan key
Look under and just to the left of the thermostat. JK
In the V between the engine blocks and under the plenums.It is a COMMON source of coolant leaks.
there is a frost plug in the back of the heat that is bad.
Under intake manifold(left side of shortblock).