Are you looking at tracking a company vehicle? Or for the protection of yourself? Or do you wanna see where you kid is going? What are you trying to do... cause I did a crapload of reasearch about a month ago.
We ended up ordering the system from Advantrac Tracking Systems. http://advantrack.com/ . We mounted them behind the stereos in our trucks at work.... and pull the data off of them every Friday. Basically lets you know how fast they were going, where they went, how long they were stopped, max speed... stuff like that for me to bust my dumb ass drivers .