All GPS tracking devices need to be able to receive at least three GPS satellite signals that are strong enough for the device to calculate its exact location.
The stronger the satellite signals the better, so it is critical to place the device where satellite signals can easily reach the device. Yes, satellite signals can bounce around inside the car or even off a road surface to places underneath the actual vehicle, but unless you have the time to do a some testing to see how a location will work, the following options will very likely work straight away with little placement time.
Inside the car
Satellite signals will not penetrate metal, but they will easily penetrate glass, plastics and particle timber products. The ideal position inside a vehicle is somewhere underneath the front or rear windscreen. Under the front dash or underneath the rear parcel tray on a sedan are ideal locations.
The driver of a vehicle might use the glove box, but if its design is such that you can't easily see the top inside of the glove box, then that might be a good location to Velcro a small GPS tracker. Alternatively, place the unit somewhere behind the front centre consul.
Most rear parcel trays are easily accessed via the boot / trunk of the car. The parcel tray will likely be a criss-cross of metal bracing with a fibreboard and carpet top. If you can place the unit between that metal bracing and the parcel tray, the satellite signals will easily penetrate the fibreboard, carpet and glass above it.
Although not ideal, another possible location is under the front passenger seat of a car. Place the unit as close to the front of the seat and the windscreen as possible.
Outside the car
Most newer cars these days have a plastic or fibre-glass cover over the metal framework of the front and rear bumpers and this is the most preferred location . You will need to secure the unit by first placing it in a case with a strong magnet or with tape, zip-ties, etc. Be sure to make sure the unit's inbuilt antennae is facing upwards and that the unit is not placed underneath the metal bumper framework.
GPS trackers with an external antenna lead can be placed almost anywhere as long as the actual antenna is placed somewhere it will receive strong satellite signals.