Ifixit and Wired got together to do a teardown of an FBI tracking gadget: "The device was loaned to us by a person who found the device on their car, and is similar to the one Yasir Afifi recently found underneath his own vehicle. The hand-assembled device is comprised of a GPS unit for receiving the car's position, an RF transmitter for relaying your location to the interested authority (aka the FBI), and a set of sweet d-cell lithium batteries that power the whole enchilada. But we didn't stop there, of course."

Tracking device teardown [iFixit]