If you are talking about the ones right under the front bumper. The black plastic pieces. I found one on eBay for a good price and popped it right back in place.
It was quite awesome. Just search in eBay "Jetta grille vent" and a list should of various pieces should come up.
This video will show you how to replace the grill on your 99-04 jetta. http://www.youtube.com/watch?v=14G3sMJ71Wk
facing engine #1 left to Right ie #1 on the passenger side #4 on the drivers side.
I made a video on how to change the head lights in my jetta This is for a 2006 jetta tdi, but other years apply http://www.truezombielife.com/2006-jetta-head-light-replacement/
The best replacement headlights for a Mk6 Jetta are the black LED Projector Headlights.
under the back passenger seet
The 2009 Volkswagen Jetta takes fairly specialized wipers. See related links below. 24" drivers side and 19" passenger side with special attachments. This vehicle also came equipped with beam style blades so its recommended to stick with that style.
eBay
Autozone $150.00
under the passenger side rear seat.
The 1998 VW Jetta drivers side power window relay switch can be found inside of the drivers side door. You will need to remove the drivers side door panel in order to access the power window relay switch.
assume passenger side is from usa perspective, i.e. right hand side. plug/cylinder 1 is closest to the passenger side. plug/cylinders are numbered 1-2-3-4 passenger side to driver's side. firing order is 1-3-4-2. Or: http://autorepair.about.com/od/enginefiringorders1/
The speed sensor on a 98 Jetta is located near the brake setup. It is on the passenger wheel area mounted to a bracket.