The block heater cord on a 2003 Ford Taurus (assuming it has one, as not all models did) is typically located in the
front of the engine compartment, near the driver's side. Look for a cord emerging from somewhere near the engine block itself. It might be tucked up near the firewall or along a frame member. It often has a rubber grommet to protect it where it passes through the body.
Because of its location, you may need a flashlight to see it easily, especially in low-light conditions. If you can't find it readily, a shop manual for your specific year and engine could provide a diagram.