I purchased a DriveOK vehicle tracking unit last week for my daughters car and I absolutely love it. It was very easy to install and is much lower than comparably priced models.
It worked better than expected last night. I hadn't heard from her when she was supposed to check in so I went online to see where she was at. Turns out her car broke down and her cell phone had died. I was able to get to her pretty quickly and make sure she got home safely.
Parents, please install a GPS device in your teens car so you can at least have that piece of mind in knowing where they are.