SSIDDetector - This is a simple mechanism I use to track when my cars are nearby the house.
While GPS detector work great, for when you have a minute or two to detect the change, SSIDDetector can be picked up within seconds. Which is ideal when you are driving down the driveway, and want the front lights to turn on.
You can use any cheap portable router, that you can plug into your car. I've used it quite sucecssfully with the HooToo and TPLink variety of models.
Depending upon how you use the car, I would suggest fixing one of your 12v adapters in the car, to be permanently on.
Each device is given it's own SSID to track, so track as many cars as you wish.
SSIDDetector link:
https://drive.google.com/file/d/0Bzupdt ... sp=sharing
NEW LINK! If you couldn't use theprevious link, try this one instead.