Is there a way to track my step-child (without her knowing) by using her cell phone?

She's fifteen (just turned in October) and stays with her EXTREMELY over-indulgent grandmother (a long story in and of itself, but not the issue at this point). I've had two neighbors call and tell me they've seen her meeting what looks like middle-aged men at the end of our road (many times, now), and she frequently is not where she is supposed to be (or where she says she is going). I need a way to know where she is at all times, for her safety as well as my own sanity. I am terrified that she will disappear into a car, only to be found dead, or worse, somewhere. Her grandmother insists that she knows everything the girl does, but I've already found inappropriate pictures on her MySpace, and other sites where she gives her personal information out freely, and invites ANYONE to call or text her. Her grandmother does not monitor her emails (or any web use, for that matter) nor will she ist anyone else in doing so (nor has she ever done it, to my extreme dismay). She also does not check the girl's phone for inappropriate texts/calls/pictures, so there is truly no telling what is going on. Add to that, she's missed close to 6 full weeks of school already this year (mostly owing to her grandmother's indulgence and laziness). I need to know what she's into and want to avoid confrontation until there is actually proof that she's doing the very things she knows she is not to do. The child and her grandmother have a bad habit of lying through their teeth - grandmother lies to protect the girl, saying she doesn't want her to be mad at her later, and the girl lies, knowing grandmother will do so to keep her out of trouble. Any help here?