2003 mustang v6 ECT location?

V6 CHT diagram

Note, late model V6's do not use an engine coolant temperature sensor (ECT). Instead uses a cylinder head temperature sensor (CHT).

Ford made this change because it is difficult to remove all of the air from the V6's cooling system. The ECT will give LOW readings if in contact with air. This leads to overheating even though the dash gauge reads normal. The CHT is not affected by low coolant levels or air trapped in the motor.

Here's the picture in the Ford Service CD