I just installed a new radiator in my 82 300D. I took this opportunity to flush my cooling system and drain the block etc. Some chunky stuff came out of the block drain plug when I was flushing the system out. I replaced the thermostat and my hoses. Now the car is leaving a coolant spot on the road on the passenger side towards the back of the engine every time I park it. I can't see where it is coming from. None of the hoses are leaking. I did start smelling coolant when the heat is on. I am thinking that the heater core is leaking. If it is where is it draining through? Also I really don't want to use the pellet type stop leak or anything since it is a brand new radiator and it is running great now. So my questions are:

How hard is it to replace the heater core?
Where could the leaking coolant be draining to get outside the car?

Thanks alot for the help.