If you have to use your laptop while in the car, let some air in by turning on the air conditioning or opening the windows. Not only will the laptop overheat, but even you will also not feel comfortable working in such a high temperature. The internal heat generated, combined with the surrounding heat, is not suitable for your laptop. Also, you might get sweaty and have it dripping onto your laptop, and it is common knowledge that moisture and electronics do not interact well.