Until the baby is a year old, their carseat will be rear facing. In that configuration, you only use the lapbelt in the midde of the rear seat. Once they are a year, and you turn their seat around, most forward facing car seats either use the lapbelt and the tether, OR the LATCH system. The LATCH system is a short lapbelt looking thing with safety hooks on both ends. Usually there are rigid D-rings mounted right where the seatbottom and seatback come together. You may have to push your fingers between teh back and bottom just a little to expose them. Check out your Jetta, I'm POSITIVE it would have them, but your 99 won't. Ther should be six attachment points in the backseat of the jetta. and Probably three tether anchors up on the back deck.
As for an anchor for the tether, you can buy them at most "baby" places. it's just a curved piece of metal with two holes in it a bolt, nut and a couple of washers.
On my 99, since I don't have the LATCH setup, I use the middle lapbelt, and then installed one of those anchors in the floorboard right up against the back wall of the cab. It was a pain to fish the tether down behind the seat.
I have a buddy that got the LATCH mount points from Mopar for his 01. Not sure on part numbers or how they attach, but I know it can be done. LATCH is simpler to install a carseat with and to remove it. I also believe it's a stronger setup.
BTW, check local laws. Read the carseat owners manual, and DON'T use both the TETHER and the LATCH system. Pick one or the other.
Hope that helps.