I have a dashcam that records on to an SD card, has two forward facing cameras and a reverse camera.
The unit has a mirror which clips over the existing rear view mirror, making it very easy to install and unintrusive. The rear camera sticks to the rear window with double sided sticky pads and is very small and light. It connects to the front unit by a wire. Took less than 10 minutes to install the whole thing, the cables are completely hidden.
It has a 32GB SD card and if there is an accident it locks the video so it can't be written over.
When I reverse the car I can see the rear view camera image in my mirror.
For the money it is a good piece of kit. I am very impressed with the build quality
http://www.amazon.co.uk/E-PRANCE-Rearview-Cameras-Degrees-G-Sensor-DVR/dp/B00FDQ93WM