I first stumbled across this on DIY Tripods, who originally found it on Reddit. The beauty is in the simplicity: Place your GoPro camera between the seat and the headrest and collapse it until snug. Then hit record.
In the DIY Tripods video (below), the camera is placed without the housing between the headrest and seat. I found that keeping the camera in the housing allowed me to nudge it slightly more forward, keeping the seat and headrest out of the image for the most part. Either way, this is one of those set ups where you’re not putting much thought into it – so it doesn’t really matter – and as you can see below, the footage still comes out looking great.