You're probably going to hear different opinions about this topic (search the archives for more info), but in my case when I added factory fogs to my '99 everything was plug and play.
You will need:
1. fog lights
2. fog light relay
3. headlight switch with fog light pull-out option
See if the original threads are still there then let me know if you have any more questions.
EDIT: I just re-read your post. FIRST, check if you have the wiring (I'm pretty sure you do, but check anyway). You'll be able to see it once you take out the covers.