You can purchase dashcams which have a g sensor, so when it detects a collision, it'll turn on and start recording. Of course you need one for the front and rear. Also depends on if your area is safe enough to leave dashcams in the car. I know mine certainly isn't
Doesn't seem like my area is good enough to leave a parked car. Let alone one with a camera in it
Thought the motion sensor thing would be available Which is why i started "Not sure...."