A jammer for a GPS (Global Positioning System) prevents devices from receiving or transmitting signals. They work by disrupting frequencies and therefore interrupt any tracking device that may have been placed in your car.
However if you're seeking to do this it is important to know that GPS radio signals use a specific frequency. There are two major frequencies that GPS work on: one is for public, non-military use (1575.42 MHz) and one that is used solely for the US military (1227.6 MHz). Most jammers available simply plug into the cigarette lighter in your car.
With the increasing popularity of cheap GPS-based navigation and/or hidden tracking devices, many feel this "security precaution" is desirable, even necessary.
Here are some links to gps jammers available:
GPS signal Jammer http://www.thesignaljammer.com/gps-jammer.html
Car-use gps jammer / blocker / interceptor http://www.gpsjammers.net/gmc07.html
Tiny Anti Tracker GPS Blocker http://www.chinavasion.com/product_info.php/pName/tiny-anti-tracker-gps-blocker-in-car-use/
Jammers for GPS http://www.alibaba.com/showroom/GPS_Jammer.html
GPS and GSM jammer http://www.navigadget.com/index.php/2007/01/29/gps-and-gsm-jammer
** It should be noted that most GPS jammers are illegal to build or use in many countries or localities due to the potential for their misuse. For instance, a GPS jammer can confuse aircraft and other vehicle instruments and therefore cause accidents.
