Generally, dash cams aren't built to sit behind the grill. You risk debris hitting the car, which will inevitably damage the camera. The wind at 70mph, as well as the heat of the engine, aren't things that most dash cams are built to deal with either. On top of that, you'd have to regularly clean it and make sure it was secure or you risk losing all your footage, and possibly your camera along with it. That being said, you could - in theory - use a motorbike camera that was adapted for what you want. The INNOVV C5 could be an option, but fitting it may be more hassle than just getting something more discreet for inside your car - like a mirror-mounted dash cam.