I print them mirrored on clear static-cling vinyl and then overprint them with white ink. You need the white, or the image won't be visible against the dark interior of the car.
The white vinyl laminate will cause them to curl over time and then, due to their stiffness because of the extra thickness, they will no longer be flat enough to cling to the glass.
Edit> If you are using clear adhesive vinyl then laminating with white should be okay.