[boschmann]

I've never seen a blocked core, usually the core will just leak coolant into the floorboard when they go bad. More than likely it's the flaps in the air distribution box. They are metal with large perforated holes that were covered with a foam material that falls apart by now. You probably have seen some of it blowing out of the vents. When this happens you only get weak heat (even in the cool position). I always replace the material when I do a heater core on them, I use foil furnace tape to close off the holes in the doors. You can also check that the temp control lever is still hooked to the flap control arm at the bottom of the air box, if you hear the flap moving then it is probably okay.