I've got the front and rear camera from BlackVue (650GW-2CH), but if I was buying now I would go with the Thinkware. Records at a higher bitrate so video quality is decent. Better than the BV from what I've seen.
Are you wanting parking mode features, of just hardwiring it to turn off an on with the ignition? Both are simple enough. Would certainly recommend doing it yourself rather than paying, although mine was on an S5, so not sure how different it is on an A6...?