How to program a 98 GT Convertible Keyless Remote
Note of thanks to rande111... his programming instructions are correct for a 1998 Mustang GT. Here are a few more details which may help you...
What has already been stated:
1) Turn ignition ON.
2) Temporarity "jump" the 2-pin connector (door locks will cycle).
3) Press any button on 1st remote (door locks will cycle).
4) Press any button on 2nd remote (door locks will cycle).
5) Turn ignition OFF.
The 2-pin connector is tricky to find. It is in the trunk, it is on the driver's side quarter panel. It took me 20 minutes to find it so here are more details. The 2-pin connector is mounted on the SIDE. You must temporarily remove the pressed-fabric by bending it way out of the way to get to the connector. The 2-pin connector is located next to a rather large white/plastic connection point having several wires extending out from the bottom. The 2-pin connector is a plastic sheath with 2 wires extending from the bottom and 2 bare "posts" within the well of the plastic sheath. To temporarily "jump" the posts, I just stuck a screw-driver in there. Immediately, the characteristic "door lock cycle" was observed and I knew I'd hit the jackpot.
Hope this helps.