Because of the dual zone a/c-heating system, instead of using rubber Ford chose to use steal or aluminum tubing from the front to the back.
More than likely the tubes have made contact with metal and with normal driving conditions have rubbed a hole in the tube. maybe just a leaky connection to the heating core or worse a heating core with a hole in it.
Plus i don't know if it is true or not but using two different kinds of antifreeze can compromise the entire cooling system (heating system too).
Fluids in a vehicles must be maintained to keep it running well. Coolant leaks because there is a crack somewhere, a tube is torn, or the clamp is too loose.
The 2003 Ford Expedition has 64.3 in. of rear shoulder room.
The 2003 Ford Expedition has 39.8 in. of rear head room.
The 2013 Ford Expedition has 39.1 in. of rear leg room.
The 2014 Ford Expedition has 39.1 in. of rear leg room.
The 2001 Ford Expedition has 62.3 in. of rear hip room.
The 2010 Ford Expedition has 59.9 in. of rear hip room.
The 2009 Ford Expedition has 63.7 in. of rear shoulder room.
The 2011 Ford Expedition has 59.9 in. of rear hip room.
The 2012 Ford Expedition has 63.7 in. of rear shoulder room.
The 2014 Ford Expedition has 39.8 in. of rear head room.
The 2013 Ford Expedition has 59.9 in. of rear hip room.