Warning: If your car is under lien, the lienholder may flip out when you disable the GPS. That is their protection to make you pay your bills or take your car if you default.
Screw drivers to remove the trim under dash.
Step 1: Locate device
I located the device using several key facts:
The device needs a clear shot to the sky to receive the GPS signal.
The device needs clear shot to receive and broadcast its status signal (through GSM)
The device works by interrupting the starter signal. (Therefor located near the ignition)
Humans are lazy and complacent.
Gain access to the underside of your dash and look for anything out of place. Look for new wiring, new electrical tape, wires that are "thicker" than thin stock wires" and wires that go off to the roof of the car. You are looking for a plastic device thats about the size of a deck of cards with two wires running into the ignition wires.
Don't be afraid to remove electrical tape and zip ties, but be careful not to break or cut the thin wires that belong to your car.