Transmission Control Indicator Lamp (TCIL)
WARNING: This page does not describe the selected car, but rather 35 other vehicles, including the 2007 Mercury Mountaineer, 2007 Mercury Monterey, 2007 Mercury Montego, 2007 Mercury Milan, and 2007 Mercury Mariner. However, it is still accessible from the selected car via links, so may be relevant.
The TCIL is an output signal from the PCM that controls the lamp on/off function depending on the engagement or disengagement of overdrive.