The act of spying with video surveillance devices can raise some tricky ethical issues, but there are some times when what goes on in your own car is your business. A sneakily installed spy camera in your vehicle can be a great way to catch a car thief or just keep an eye on how other people are using your car (and who's driving in it). The close quarters of a vehicle make for a good place to easily catch sight of the people in it, but also create challenges for effective concealing of such a camera.
Buy the smallest and most portable camera possible. There are hundreds of different tiny, battery operated spy cameras available, including many that are already disguised as other objects. Check out you local electronics store for basic offerings, or look at sites like the Spy Review Blog (see link in "resources") for the more speciality offerings. As you buy, be sure to consider factors like length of recording time (how much time will you need to spot whatever it is you're looking for?) whether you will want an Internet feed, and whether you will need your recording to have audio. In general, the cameras that are the smallest (and therefore, the easiest to hide) will have less power and the fewest extra features.
Choose a good location for your camera based on what you hope to see. For example, if you're hoping to catch a glimpse of a car thief's face, you'll want your camera to be positioned somewhere on the dash board, facing the driver's seat, whereas any shenanigans you hope to spot in the backseat will require a camera positioned somewhere in the back of the front seats. While it may seem like a good idea to get a camera whose angle and focus can be remotely adjusted, bear in mind that this will likely make a sound that will be noticed by whomever you are trying to spy on.
Disguise your camera in an object that can be removed from the car. This is the easiest way to hide the camera, but you'll want to choose an object and position that the passengers will be unlikely to move. Good options include sunglass clips on the overhead flaps, hulu or bobble-head sculptures on the dash, large objects hanging from the rearview mirror (like fuzzy dice) or reusable trash bags that hang behind the front seats.
Hide your camera somewhere in part of the car itself. This method will take a bit more work, but if you can pull it off successfully, it will be the most secure way to hide a camera that won't be noticed or removed. Try hollowing out a space for it in a seat cushion (a seat cover made of a translucent, stretchy fabric will hide this well), inside the dash board, or inside vents or speakers.
Test the positioned camera to see what it's recording. Even if you're sure you've positioned it well, you will want to take some test footage (or perform a test feed) to make sure you're getting the view you want. If possible, test with people in the car to make sure you're catching their movements and have the camera adjusted to proper head heights.