One thing I detest about today's cars (this means anything made after 1989) are that all of them have plastic lensed headlamps. While there are a few exceptions that have glass, mainly the early 1990's Buicks, most are made of Lexan. This is tough, durable, and allows for styling than the old fashioned sealed beams (which are only available on GM full sized vans at this point). When a seal beam breaks, you replace the assembly with a nice, new clear one for less than $20. With one of these plastic composites, you have to spend over $350 in some cases to replace the assembly; which is a laser welded unit.
The problem with these lights isn't so much that they break, but they yellow within about five years. Sure, there are commercial kits to deal with this, but these are $5 to $30 and this writer has yet to see one work. This idea isn't all mine, but I'll share it and can attest that it does work. It costs about $10 to $15, depending on what you already have in your shop. The results are dramatic, and long lasting. Depending on the quality of your work and patience, these will look brand-new and will have their optical quality more or less restored.
I might add that you need to make sure there aren't any laws concerning this where you live. I would also work in a clean, dry and well ventilated area, especially for the finishing stages. Common sense would tell you that if there are other issues, such as broken mounts or a header panel (in the case of my wife's car) these need to be addressed before putting the car back in service. Headlights will also need to be aimed as the optical quality will be different. If these are in poor condition, consider replacing with new or salvage units and restoring the latter or just installing the former. I've used Goop adhesive from everything to car door panels to seat fabric and heater control and if you have the time to let this cure, it works fantastic. The red tube costs about $4.00 and dries harder than the plastic it's bonding, but I digress. Since I have no control over the quality of your work, you this at your own risk.
You'll need one or two each sheets of wetordry sandpaper, 400 and/or 600 and 1000 grit, some Dawn dishwashing detergent and a can of good quality acrylic clear coat. Depending on how hard it is to get the headlights off the car, some masking tape, towels and a spray bottle will be necessary. In that case, fill the bottle with water and add a squirt or two of dishwashing detergent. If you can remove the lamp, just use a sink or bathtub lined with an old towel or two. Add some water to the tub or sink and add the detergent. Wet the lens and sand with the 400 grit in one direction with the width of the lens, until it is uniformly sanded. This will take time to do, so take breaks. The ides with the detergent and water is to lubricate the lens while you sand it. Once you've got all the yellowing off, the lens will have a milky white appearance and all the clearness will be gone. This means that you're ready to switch to a finer grit. You can use 600 if you want; and this will make the final sanding with the 1000 a lot easier, but it isn't mandatory. Remove the lights and take out the bulbs of you want. Best to leave them in to help seal the light up, but this was not an option on my wife's Grand Am.
Sand with the 1000 until your are satisfied that the scratches from the last grit are sanded out. The appearance will be slightly clearer, but these will be finer scratches. Thoroughly dry and inspect the lenses for any touch ups needed. If there are a few stray scratches, I wouldn't worry about it. Just take your time for the best result. If you have any water inside the lens, use a hair dryer to get this out and let it air out for a few hours. Once this is dry, you can take these to a suitable area to spray on the clearcoat. This should be done on a calm day, with minimal dirt and dust. Spray in light coats in five minute intervals to prevent runs that would need to be sanded out. Three or four light coats is better than one heavy one, and the lamps will have a better optical quality too. Let this dry for a few hours before putting back on your ride. Re aim and enjoy a clearer view. Maranatha!