This isn't really the right thread to say this, but if you have an older car dirt sometimes works its way into the inside of the
headlight housing. You can remove the whole assembly, wash out the inside with soapy water/alcohol, dry out, and reinstall. Maybe follow the soapy water with alcohol since the alcohol will absorb the water and dry out quickly - common practice in lab glassware cleaning. This can make a big difference depending how dirty they are.