The headlights on the S40 were very bad when we purchased the car.  I tried using several brands of polishing compounds to bring new life to the lights, it did help, but it was still really dangerous to drive at night.  It looked like the plastic had thousands of little cracks going all the way through the plastic.  I purchased a new set online, I think Amazon.com.  It is amazingly easy to remove the headlights in this car, just pull one pin up and they pop out.  They have been on the car a few years now and are still holding up pretty well.  Brand is TYC.

Products used:

• Two off-brand headlights
  • TYC 20-6857-00
  • TYC 20-6858-00