Have you used a bright flashlight to just look up under the Expy and see what you can see ? The heater lines and the AC lines run right under the rear deck in that area. If it was the heater core it would most likely run out the drain hole at the rear where the lines go in.
That doesn't look like Ford Gold coolant, so who knows what's been used in there. Do you have a borescope camera ? But when it gets done dripping you can lay under there and see with a flashlight what the lines look like. If it's in a section of the heater hose you can probably put in a brass splice piece from Home Depot along with some hose clamps.
If you've had problems with the rear actuators it's a good time to replace those too. But this is what's under that quarter panel.
But this is the rear heater core and you can see it's pipes exiting the HVAC box above....
