I have a Chinese dash cam on mine,
It was going to wire it into a switched live but in the end I just used a long usb lead to power it from the USB port in the glovebox (I never use it for anything else anyway)
I stuck the camera behind the rear view mirror (on the passenger side of the screen) so it doesn't distract me/get in the way fed the cable up over the sun visor and then under the rubber stip down the door edge. Thought the glovebox and into the USB port, I had to blank off the 2 contacts in the middle of the USB connector (so the camera doesn't think it's plugged into a computer, it just sees a power connection rather than a data one, I just folded a bit of paper over the contacts)
The camera comes on everytime I open a door or turn on the ignition so I dont even have to think about it.
It hasn't let me down yet and cost me less than £15 all in (including a 16gb memory card)
If anyone is interested drop me aline and I'll put all the facts and figures into a tutorial.
I also bought the same cameras for my Wifes Captiva and her parents 3008, the instals are all much the same so the cameras start recording when the ignition is switched on they record for about 1 1/2 hours in 3 minute chunks (on a 16Gb memory card), then start to overwrite the oldest chunks. you can either unplug them or press a button on them to stop them overwriting the current recording in the case of an accident, they also record audio so if you do have an accident you know what you were talking about at the time. (might be useful to prove you weren't on the phone?)
Axe