If you own a GM vehicle (God help you) with OnStar, and you are fairly good working with electronics, this how-to guide might be of interest to you:
"Tap into OnStar"
"Tap into OnStar"
Any Vehicle with OnStar installed has a GPS reciever inside that is always on, ready to tell you exactly where you are. The problem is, the only way to access this information is to call onstar, wait for them to answer, wait for the modem to upload your location, wait for them to read it back to you... and all the while they charge you a hefty service fee.