Is the leak inside the passenger compartment or in the engine compartment? The dashboard (firewall) divides the two.
The easiest fix is to bypass the heater (connect the long heater hose to the back of the cylinder head). It usually solves a leak unless the fitting is rusted out.
You may leave it this way temporarily or permanently -- the engine runs fine either way. Just depends on whether you need the heater for de-misting the windshield.