As the other in car camera thread seems to be pretty much based around the 'torch' type DVRs, here's one for a different type - the HD type with inbuilt 2.5in LCD screen.
These are the units concerned (this is just one seller - there are others) :
I have had one mounted in my Vectra for a couple of weeks now and I have to say I'm pretty impressed. None of these date/time files, all settings are done on the unit using the menus on the screen.
It's mounted behind the passenger side of the rear view mirror and the power cord is currently run around the edge of the windscreen (I may sort a more permanent power supply later) although it does have an internal battery too.
When powered via the aux socket as soon as the socket becomes live (turning on ignition) the DVR switches on and begins recording. When the socket is switched off (ignition off) the DVR saves and shuts down. No forgetting to switch it on then finding it was off when an incident occurred.
It takes full sized SD cards up to 64Gb capacity, and like most when full it starts recording over the earliest saved files.
Here's a few pics to give a better idea of one actually fitted (Preface Vectra):
Screen flipped down or you'd not see the unit at all.
Ok so it's not a brilliant pic to show the DVR from the outside.... but to be honest unless you look very carefully you can't see it anyway !!