After ALL this work...NEW heater core also LEAK!
 

Changed my heater core over the last two-three days. Replaced the old one with a brand NEW acdelco heater core. But what the F, there are still some coolant dripping on the passanger floor side. I suspect that the O-ring on one of the pipes going to the engine bay is where the leak comes from. The one pipe was looser then the other when I checked it out before installing the heater core.

Have you checked and confoirmed it yet? Or is the leak somewhere else?

Yes I have checked it and YES there is a leak in the new heater core. The leak is at the O-ring from the one pipe going from the heater core and through the firewall. And this is no aftermarket heater core, it is AC Delco.