Like most higher-end production cars rolled out since the mid-1990s, the 1999 Mercury Mountaineer is equipped with a keyless door entry. Reprogramming the door entry may be necessary when buying a used Mountaineer or if you simply forget the code. The five push-buttons under the front driver's side door handle contain the digits 1 through 10, with two digits on each button. These buttons can be set to unlock the Mountaineer with any four-digit combination: 1-2-3-4, for example.
Open the driver's door and leave it open throughout the programming process.
Insert the key into the ignition. Turn the key to "On" then "Off." There is no need to start the engine. Repeat this on-off cycle eight times in 10 seconds, leaving the key in the "On" position at the end of the eighth cycle. This will initiate the Mountaineer's programming function. Note that the Mountaineer's power locks should unlock then lock automatically to indicate initiation of the programming function.
Press a button on each of the keyless remotes then quickly enter the four-digit code on the outside of the door panel.
Turn the ignition to the "Off" position. Note that the power locks again should cycle to unlock then lock. This indicates the Mountaineer has accepted the programming.
The Mountaineer also comes with a manufacturer's default code, which is included in the owner's manual. This code also can be used to lock and unlock the vehicle.