If your heater core is causing puddles of water under the car, you should have water inside the car under the floor mats as well. You should be able to send the coolant when you're driving around.
Maybe one of the hose connections to the heater core at the firewall is leaking???
The heater cores of the W201 are known for an occasionally leaking one...I exchanged several in the past and have never tried a coolant additive to close a leak in the system...
with a minor 600+k