Does this happen when you have the heat on or the defroster?
Happens when the heater is on... not the defroster... doesnt seem to matter which one is on. Its as if the thermostat closes up when the heater is on...which may be the case given the heater hoses exit from this same area. Is this a reverse flow engine? Thats the only thing that makes sense with the thermostat in this location because if this is the cold water inlet it would just close the thermostat. Do you have any way to check that this is indeed the thermostat location because all the heat sensors are at the back of the head...I just bought the car so I have no history.
i will post a diagram of the thermostats location for you. Can you tell if the ac compressor is coming on when the heat is turned on? You may have to look at the ac compressore and see if the clutch is spinning with the enig running, and the heat on.
The thermostat does gointo the lower radiator hose housing on the engine. Here is a diagram for you,
If the ac compressor clutch enguages, see if the electric fans come on as well please.
I will be right back, I have to go and pick up a customers car. Be back around 3 pm. It is 2 now.