If yer car has leather I'd suggest putting non-slip tape on the back side of yer phone. If it's cloth then scuffing would probably work??
Just lightly scuffing with a wire wheel grinder makes it incredibly grippy. I have an extra Sheath, It's slippery but not as bad as most cases are.
After a couple of weeks, the rough feeling groves smooth out a little actually making it feel better in the hand but still provide great grip.
I would rate the grippyness of a stock Sheath around 60-70%. Apple Silicone 50-60%. Modified Sheath 90-95%.
I didn't scuff the back only the left & right side on the curved part. It was all I needed because of the way I hold cell phones.