Glass blocks some solar energy. If there's a glass layer over a solar panel, it has to be a special variety known as low iron glass, and windshield glass is not the same. However, if you are trying to charge in your car, the more likely culprit is the heat inside the car. Solar panels get less efficient as they heat up (part of our reason for designing the panel to be very thin). If it's in an enclosed space like a car with nowhere for the heat to escape, that could slow things down considerably.