buy a dashcam that has "knock" or G sensor to power itself up and take a pic. Most new dashcams have internal batteries so would work for this. Mine does. The G sensor you can set the sensitivity, when I unlock my car and get in the camera is already powered up before I put the key in. The G sensor also, I'm sure people are aware, takes a snapshot when in an accident as such. Great things, but my only advice is to put the smallest sd card in possible. I think the 8GB cards are no longer sold so snap up the 16Gb quickly.
I only say this because the less that is on it at any given time, the less 3rd parties have to look at and your latest history of driving etc etc. But is an invaluable asset if you are not at fault for a prang!