Daniel Davies has made an interesting Touch Sensitive Car Security Device called the G-Spot. With this device installed turning the key is not all that is required to start your car. You also need to know about the secret G-Spot location, while touching your grounded key you also need to touch the G-Spot location to successfully start your car. You could imagine that a metal screw on your dash could be the secret touch location that a crook would probably never know to touch. Daniel has provided full documentation so that you could build your own, he has also gone over some of the steps he took to get his system patented in New Zealand.
Thanks for sharing your design with us Daniel and good luck with getting it to market.