Old thread, but took the Mustang out today for a drive & wash. I drive the car every 3 to 5 weeks, about a 10 mile circuit (give or take) to keep it oiled up & running. As I was driving, I thought 'that smells like coolant'. Got to the car wash and saw coolant dripping from passenger side floorpan. So after only about 300-350 miles (and that's a generous guess, but I have to guess since the speedometer & odometer aren't functional) the heater core is once again leaking (badly) and has to be replaced. I've changed the coolant twice in the meantime too, so doesn't seem like that should be eating it up. On the plus side, I never fully reinstalled the dash, due to other ongoing work.
best
Hank