1. See if the OBD interface on the car is plugged in. Some GPS can be plugged directly into OBD interface.
2.GPS system needs power supply, so you can find it by checking the circuit. But now there are many GPS without wiring and equipped with strong magnetic GPS locators. You can buy GPS shield from a treasure.
3. You can check the ceiling lamp above the front windshield, the hidden place in the decorative plate below the front windshield, the hidden place around the front instrument panel, the door partition, the decorative plate below the rear windshield, the front bumper and the wiper blade.