Vehicle headlights, whether plastic or glass, accumulate dirt, debris, stains and damage that create a film that blocks light beams--shortening the range of the beam and/or angling the beam in a less optimal direction. Whether you have a cloudy or yellow headlight lens, or a lens with interior condensation, in most cases you can quickly restore your headlight lens to new using a few easy-to-find items.
Wash the headlight lens with a mild detergent and a soft-bristled pad or brush. Rinse to remove any debris.
Dampen your melamine resin foam eraser and gently rub a tiny spot on the corner of the lens to check if the eraser noticeably scratches the surface of the lens. If it doesn't, gently rub the eraser over the headlight lens. Melamine resin foam erasers have a hard structure similar to fine sandpaper and can rub away any fine particles, stains and debris as well as smooth out deep scratches. Use toothpaste and a scratch-resistant pad as an alternative if you don't have a melamine eraser. Toothpaste not only removes stains, but contains silica which acts in the same fashion as the melamine resin.
Rinse away any residue and apply plastic or glass cleaner to the surface using a soft, lint-free microfiber cloth. If some cloudy or yellow film remains, use headlight lens plastic/glass polish and a buffer or a headlight restoration kit.
Apply a headlight protectant (liquid or film) to reduce future discolouration.
Search for cracks or holes in your headlight causing condensation behind the lens. If you find more than one, or a large damaged area, take your vehicle to a mechanic to look at the damage and/or have the headlight replaced. Police equate certain types of damage--a "broken" headlight for example--as a ticketable offence.
Clean the headlight thoroughly so that you can gauge the amount of condensation.
Dry out the headlight with a hair dryer if only a small amount of condensation exists. If the headlight has a lot of condensation, go to Step 4. Place the hair dryer one inch from the crack or hole and run it on the low setting for 15 minutes. Repeat until dry, taking care to not melt any plastic or damage your vehicle's paint, and then go to Step 5.
Remove the headlight. Drill a tiny hole (2 to 4mm), also known as a "drain hole," through the bottom behind the lens and drain out any water. Dry with the hair dryer or, if you're not using your vehicle, set the headlight aside in a low humidity area for several days to dry out.
Press clear headlight tape over the crack or hole--making certain to remove any bubbles as you press--to protect against future condensation. If you created a drain hole, cover with tape, bond to the headlight by heating lightly with the hair dryer and then place the headlight back in your vehicle.
Always take your vehicle to a mechanic after Section 2, Step 3 if you don't have headlight removal auto repair experience or find algae or dirt inside the headlight; your inexperience could lead to headlight damage. In addition, seek professional help if you can't find a crack or hole causing condensation. Your headlight may have a seal leak requiring professional repair or headlight assembly replacement.