There are a lot of different options of in-car camera mounts. Some have the camera mounted to the windows, others mount the camera to the dashboard. But the position that I like the best has the camera mounted to the headrest.
This design is sturdy that doesn’t require any modification to your car and I also think you can get the best in-car shots out of this position.
My friend did this at home and it’s pretty simple to make. You only need a piece of lumber, some screws and a power drill.