Look at this, you can pull up the web site and locate your car any time you want. There is a monthly fee, no one is going to offer this service for free!!
I have had this on my chevelle for 2 years. You can use all the options or just the locate feature which is what I have. You can add or remove features when you like. It also has a geo fence you set around the car, if car moves outside the fence you get a call so you know within minutes if it moves.
http://www.mplusgps.com/