Dear fouadabid,
Tracker is something different from navigation system. I was talking about Tracker. As for as Navigation is concerened, It can be designed easily. There are many companies making maps. You can even use google earth to display your location and realtime movement on LCD. It wont have voice functionality untiil you have good calibrated digital maps. In US you can get such maps for almost free. Over here is completly different. Suparco is selling Satellite imagery data for 300000 for one city only. You know what, same data Google Earth is offering free to view. Not only view you can do little customization too. Voice less Navigation system can be built using Google earth and you wont need to get GPRS connected all the time to use it as it get cached.
I haven't examine navigation system in imported cars. There should not be a great difficulty to port em.
You dont need to buy costly maps from crapy ppl who want to rip you off. Use Google Earth for automated display. As for as how to get tracker programed, It comes with customized program. I know the guy in Islamabad who is fabricating such trackers.