Benjy, it has been discussed widely on the forum from both an ethical / legal and technical point of view.
Whilst it used to be possible to do this using "hacks", MB closed the door on it firmly about three / four years ago and this is simply not possible any more (if Alfie says it can't be done, you can take it that it can't be done).
From the replies, it looks like there may be a third party solution but (without going down the ethical route too far), I think that you would be putting yourself on thin ice in the event of an accident if it were discovered that you had enabled this functionality. An accident investigator would possibly use the existence of the equipment as a reason to not pay an insurance claim. You could also get into trouble with the Police if they passed your vehicle (or you passed them) and they could see the screen was displaying video. there was a lorry driver done for this quite recently (watching TV whilst driving).