Automobile Hidden Door Unlock Button
This modification to an automobile is the addition of a hidden unlock button to the automobile. The button is usually recessed in a hole in the underside of the drivers side mirror. The purpose of the button is to prevent the driver from accidentally leaving his keys in the car and not being able to retrieve them. Note that this modification could allow easy access to the contents of the vehicle if the button is discovered. An alternative is to secure a key under the frame of the vehicle, but that would allow the vehicle to be stolen and the hidden key is often easily seen by the auto service personnel.
Most vehicles have either of two types of door lock motors. One type has two power inputs and a ground, and power is applied to one power lead to unlock the doors, and the other power lead locks the doors. A momentary closed push button is wired in parallel to the door unlock switch. Usually the power is in the center, and the motor lock/unlock are on either side of the center power. The momentary closed push button switch is installed in the bottom of the mirror housing, the two wires are then routed through into the door, and spliced into the push button door unlock switch using crimp splice connectors. Chrysler products use this type of door unlock system.
However, some vehicles, including Ford, use a bi-directional lock/unlock motor. The lock/unlock switch applies power in one direction to lock the doors, and applies power in the other direction to unlock the doors. Therefore, a simple parallel momentary switch will not work. When the door unlock button is pressed, it applies power to one of the leads, but it also shorts other power lead to ground. To simulate this power switch, the momentary switch applies power to the unlock power lead, but also supplies power to a normally open single pole single throw (spst) relay. The normally open circuit in the relay is wired to the other power lead, and to ground. When the momentary switch is pushed, it energizes the relay and make the return ground path. Note that pushing the hidden unlock button while pushing the interior lock button will short the system and blow a fuse. The relay is spliced into the door unlock switch wiring using wiring splice connectors, and then black taped into the wiring harness.
A quick review of any automobiles wiring diagram would indicate which type of door lock motor is used.
Two Power Lead Motor & Bi-Directional Motor schematic drawing
Car Manual documentation