How to do it in the car without it shaking?

My bf and I have a car date planned. Dinner and a movie and a little alone time afterwards. In my car. I have blankets with clips for the windows so we don’t have to be worried about being seen. It’s just every time we do it, the car shakes like crazy. I’ve read that it’s only illegal if you’re in sight of people and we won’t be. I’m just wondering if anyone had any tips or tricks to keep the car from shaking?