Wired.com on how to find and remove a GPS tracker from your car:
Just get under your car and look around. If you see a black box, give it a yank. If it comes free, it’s probably not factory-installed. Even if you’re not sure what a normal car underside looks like, a surveillance device is going to look out of place.
I was really pumped to read this (admittedly paranoid) article, but come on — all the different options Wired gives readers are pretty pathetic. I am going to go ahead and recommend that you not climb under your car and start yanking on things.