If it has the original type of battery terminals a Jetta came with, I would suspect they have gone bad. Make sure the battery cable ends are cleaned on the inside - I use a penknife to scrape the inner surface. Also make sure the battery posts are clean, by gently scraping them with the knife-blade. With the key in the ignition and turned to the 'run' position, you should see some lights on the dashboard. If not, then a relay or a fuse is most likely bad - once you have verified that there arent any postive battery cable or negative battery cable points where corrosion has collected.
Does your car have the long row of fuses under the dashboard, with relays mounted on top of that? Mine is a Mk-2 (88-92) so may be different from yours, but on mine, behind that bank of fuses and the relays plugged in above it, is a maze of electrical connectors. I actually have two 1990 Jettas, and on one of them, some of the electrical connectors had come loose. To get the car to work again, I unbolted the drivers seat, laid a narrow sheet of plywood on the floor, so I could lie on my back, looking up from under the dashboard. It was the only way I could see if I was getting the connectors plugged back in. My car had the little 'tray' in front of the knees of those in the front seat, and I had to remove that for better access also.
And if you dont have a Bentley manual for the car yet, might as well look for one on ebay, or just buy it from Bentley.