The most likely cause is an intake manifold gasket leak (that is if all the hoses and water pump have been ruled out)
How do you change the thermostat in 1999 Mercury Villager
Yes.
According to the 1999 Mercury Villager Owner Guide : ( 20.0 US. gallons )
In the engine.
No, no room.
134-A
4.5 quart
ABS was an option. Not all have it.
In the fuel tank.
There are two.
on my 1999 ford mercury villager its on top of transmission under the van
if you can smell antifreeze you need to replace the heater core in the vehicle