From http://my.voyager.net/tmclane/

How do you reprogram the remote keyless entry transmitter?

This procedure is used if you need to program a new transmitter or additional transmitters. Programming is done by pressing the button on the transmitter after entering the programming mode. To enter the programming mode, use the following procedure:

Turn the security system off by pushing the security switch (light off).
Turn the ignition switch on, then off.
Immediately push the security switch on and off rapidly until the horn starts to beep; then stop pushing. The system is now in the programming mode and you are ready to program the transmitter. The programming mode is indicated by the parking
lights flashing and a clicking noise emanating from the glove box.
To program two transmitters slowly press the remote button two times on each transmitter.
To program one transmitter slowly push the remote button four times.
Note: The parking lights will turn on when the button is pushed and turn off when the button is released.
To exit programming mode, turn the ignition switch on, then off. The parking lights will stop flashing and the system can be used normally.