This is a very simple and cheap in car camera. See example video above.
Step 1: First and Only Step
You need to get a couple of the wide rubber bands. You will have to try a few different ones to find ones that are tight enough but don't break. Mine are tight enough I thought they might break but never did.
You need to take the headrest off the passenger seat so you can fit the rubber bands around it. Once you get both rubber bands installed put you phone in place to check fit. I used the front camera so that I could unlock and start the recording without having to remove the phone. It only 720p but that's ok with me. Storage on my Nexus 4 becomes an issue quickly when recording in 1080p. I found that I could aim the camera in a couple different ways. My Jetta headrest is adjustable up and down and also rocks back and forth. This takes care of the height of the picture. The headrest is also concave so moving the phone side to side points the camera a little (not much adjustment here but a little). I thought about putting something behind one side to point it more but I thought it might cause the phone to slide and the view was ok for me.
I have a short charging cable I keep in the car and it was just long enough to reach the phone if I moved the whole seat all the way forward. Next time I will grab a longer cable. This will also give me the option to move the seat forward and back to better frame the shot.