It does not matter what the ambient temperature is, this is the very reason for a stat I.E to regulate the temperature, so say if you have an 86 degree stat it will not open until the coolant is at this temp at which point it can get back to the rad, anything below this temperature the stat closes or slows the flow meaning coolant can not get back to the rad and so will not get cooled.

However if the stat is stuck open the coolant will get over cooled so effectively it's the same as running with no thermostat, so the engine will warm up very slowly and the coolant will not get to the correct temp hence being mirrored in the heating.

This is why the heating gets warm but never hot and when you start moving it goes cold again as the coolant is being over cooled.

Yes Focus will display the temp from CHT on the digital dash amongst a whole variety of other useful info.

Martin