Well it is not really DIY, but this device from GPS Snitch is exactly what it says on the box, it is GPS device that will enable you to track your vehicle whenever you need to, but for most people it will be of great use should the vehicle be stolen.

The device is similar to a cell phone in that you have to charge it up before us, but once that has been done you can place it somewhere safe and hidden, the charge will last for about a week so you have not got to do it all the time. You then put into the device a sort of perimeter, say five or ten miles etc, if the car is stolen and it breaches your limit you will then receive a text message telling you so, from there you are able to monitor it progress etc.

Features -

No installation required
The GPS Snitch is completely self-contained. Just turn it on, arm it and hide it in your vehicle.  Lasts up to 7 days on internal battery, or use the hardwire kit.

Low Monthly plans
It’s all you’ll need to pay to have the ultimate in GPS-based security.

Motion Sensitive
Once armed, the GPS Snitch monitors your vehicle for the slightest movement.

Instant Notification
Receive notification via email, or text message to your mobile phone when the GPS Snitch detects motion or leaves a security perimeter.

Real-time Location Tracking
Log-in to www.gps-snitch.com from a computer or your web-enabled cellular phone and locate your vehicle in real-time, at the click of a button.

A New Kind of Security
You can use your GPS Snitch account to track other GPS Snitch units as well. Simply invite a family member or friend to join your tracking list, and your unit will be visible to them when you want it to be.

The only drawback is the cost, at around $300, $30 activation fee and between $15 and $26 monthly subscription fee.

Source  [Navigadget]