Have you checked the cars thermostat?
Why is Coolant overflowing from the cooling tank at times and drains down into the left rear well?
sounds like you have a blown head gasket
Oil in the coolant tank indicates a head gasket leak.
Most likely because you have added to much coolant when it was cold.
Coolant is old.
Yes
I believe that you fill it with coolant, as the tank's name suggests.
This is one small problem with this generation Cougar, is that many of the low coolant sensors give false readings, saying that the tank is low when it is in fact, at the MAX line. To correct this, you can safely add coolant about .5"-1" above the MAX line without the reservoir overflowing when the engine get's hot.
Coolant is getting into the ATF possibly through a hole in the oil cooler
As the engine coolant heats up, the coolant expands, so the coolant needs a reservoir or expansion tank to flow in and out of as needed.
The rad. coolant tank is located on the driver's side fender area.
through the coolant tank!