With a 98 it can be one of two methods depending on when it was made exactly:
1) (unfortunately the most likely) Turn ignition to RUN or ACC. Momentarily jump pins in programming connector located in left rear quarter panel area. All doors will lock then unlock to confirm programming mode. Press any button on 1st transmitter. Door locks will lock then unlock to confirm programming. Press any button on 2nd transmitter (up to 4 transmitters total possible). The door locks will lock then unlock to confirm programming. Turn ignition to OFF. Locks will again cycle to indicate end of programming mode.
2) (if it was made late in the model year) Cycle ignition from OFF to RUN or ACC eight (8) times within 10 seconds, ending in RUN or ACC. All doors will lock then unlock to confirm programming mode. Press any button on 1st remote transmitter. Door locks will cycle to confirm programming. Press any button on 2nd transmitter (up to 4 transmitters total possible). Turn ignition to OFF. The door locks will lock then unlock to confirm programming.
With a 98 it can be one of two methods depending on when it was made exactly:
1) (unfortunately the most likely) Turn ignition to RUN or ACC. Momentarily jump pins in programming connector located in left rear quarter panel area. All doors will lock then unlock to confirm programming mode. Press any button on 1st transmitter. Door locks will lock then unlock to confirm programming. Press any button on 2nd transmitter (up to 4 transmitters total possible). The door locks will lock then unlock to confirm programming. Turn ignition to OFF. Locks will again cycle to indicate end of programming mode.
2) (if it was made late in the model year) Cycle ignition from OFF to RUN or ACC eight (8) times within 10 seconds, ending in RUN or ACC. All doors will lock then unlock to confirm programming mode. Press any button on 1st remote transmitter. Door locks will cycle to confirm programming. Press any button on 2nd transmitter (up to 4 transmitters total possible). Turn ignition to OFF. The door locks will lock then unlock to confirm programming.
I just had one done at the ford dealership a few months ago. He pulled the trunk liner on the driver side back and it took him all of a minute to program.