I don't actually thing it will need to be mandated.
Cameras and storage are so cheap, that they're already being integrated into many, many vehicles on sale today, and taking the extra step of mounting camera modules elsewhere in the car, and making the link to a storage medium doesn't sound like a big effort, even in the automotive space, which is a bit more traditional than other electronic market places.
Also, a mandatory installation would be fine, for as long as the owner of the data retains full rights over the video, and crucially can not be subpoenaed for the data. Giving acess to the tapes needs to be optional and by free choice.
Rules for commercial vehicles might be different, but a private party should not be coerced to offer potentially self-incriminating data. To assure this, the video would have to be recorded with public key encryption.