Ford 1999 Explorer Owner's Manual page 115

Controls and features
5. Within 10 seconds of turning the ignition to OFF, insert the
unprogrammed SecuriLock key (new key) into the ignition and turn
the ignition from OFF to RUN (maintain ignition in RUN for at least one
second). This step will program your new SecuriLock key.
6. To program additional SecuriLock key(s), repeat this procedure from
step 1.
If the programming procedure was successful, the new SecuriLock
key(s) will start the vehicle's engine. The theft indicator (located on the
instrument cluster) will light for three seconds and then go out.
If the programming procedure was not successful, the new SecuriLock
key(s) will not operate the vehicle's engine. The theft indicator will flash
on and off. Wait at least one minute and then repeat the procedure from
step 1. If failure repeats, bring your vehicle to your dealership to have
the spare SecuriLock key(s) programmed.
115