I know it may sound totally outrageous, but... replace the leaking radiator?

Ford had a lot of trouble with some models in the Aerostar series.  I dont
know if yours is one of them.
If you know the radiator is a problem, take it out and have it repaired or
replace it.

The leak down the backside of the engine could be a Welch (freeze) plug that
has rusted through, a leaking
hose, a leaky gasket, etc.  Often you can find these easily with a
telescoping mirror (that you can buy at an
auto parts place for a couple of bucks).
Once you locallize the leak it will be a lot easier to decide how to handle
it.