I wish you the best in your situation, but KNOWING that he is cheating may not be the solution to your problem. If you suspect him infidelity, there is a bigger problem that needs dealt with.
If you are considering using the GPS information for divorce proceedings, I'd suggest you contact an attorney and you would probably be advised to get a private investigator because your own tracking may not be admissible evidence.
If it would help you to know where he is, check out: http://terratracker.com/html/geotrak.html there are other services out that will do about the same thing, this was the first one I found doing a web search for "CAR GPS TRACKING"
Good Luck
Scott