Wired obtained a device found by activist Kathy Thomas on her car in 2005. As part of its coverage of Thomas' plight--she had allegedly been tracked by FBI for many years--the device was sent to iFixit for analysis.
Overall, the unit is fairly simple and contains four major parts: a magnetic mount, a GPS antenna, a transmitter/receiver unit, and a battery pack. Its simplicity likely has a lot to do with the fact that the FBI doesn't want you to ever find it unless you're looking for it, and something more complex would be easier to find.
The device is powered by four lithium-thionyl chloride D-cell batteries, presumably so agents can track you over time. With a rating of 13,000 mAh on each cell, the GPS device would likely be able to stay powered for 10 to 20 years. Another important consideration is making sure it stays on the car: iFixit said that the magnets are so powerful that the devices mounting brackets need to be broken off it to remove it.
Will iFixit or Wired find the FBI at its doorstep soon? Time will tell. Here are some other interesting highlights from the teardown:
• The device's antenna is actually made up of two: one for the GPS positioning, and another used to send that data to FBI transponders.
• iFixit said that it was apparent that the FBI didn't want anybody tampering with the unit: screws were coated with threadlocker, requiring a power drill to pry the device open.
• The device appears manufactured by the FBI itself, and some of the parts are fairly old: the GPS signal processing unit was from 1999.
It should be noted that the FBI may have changed its methods since Thomas' unit was installed: in 2010, Arab-American student Yasir Afifi discovered a tracking device in the engine bay of his car, but this one was directly attached to the battery. Thomas' device was self-powered, and located underneath the car. The FBI later seized the device from Afifi after he posted pictures of the device on the Internet.