At the firewall are two hoses that go to the heater core. You disconnect them and connect them together to each other thus bypassing the heater core. You can also do the bypass at the thermostat module if this is a 2.0 or 2.4 engine. Coolant will now not go into where it's leaking, but you will not have heat. Your AC should still work fine.
Eventually you will need to have the heater core replaced and then the hoses can be reconnected where they were.
The dash and HVAC housing are removed as a unit from the vehicle. Then the HVAC housing is separated from the dash and dissembled to get to the heater core. This is likely a two day procedure that will be quite expensive and should be done by someone who has done it on a Dart.
24 - Heating and Air Conditioning / Plumbing / CORE, Heater / Description
The heater core is mounted in the HVAC housing, which is located behind the instrument panel. The heater core is a heat exchanger made of rows of aluminum tubes and fins (2). The heater core is positioned within the HVAC housing so that only the selected amount of air entering the housing passes through the heater core, before being distributed through the heating and A/C system ducts and outlets. One end of the heater core is fitted with a tank (3) that includes the fittings for the heater core tubes (1).
The HVAC housing must be removed from the vehicle for service of the heater core