If the headlight lenses are made of plastic, you have to be very careful when using cleaning solvents. Although protected by a scratch resistant film, polycarbonate plastic is readily attacked by solvents such as petrol, acetone or carbon tetrachloride. Also avoid highly alkaline cleaners. Alcohol, on the other hand, is OK, as is window cleaner containing ammonia, but always wash with clean water afterwards, just to be safe.