You can quickly and easily repair a crack in a running light without having to replace the light. This is an easy DIY fix if you have a way to reach the lights. Save yourself hundreds of dollars, you can fix it yourself for less than $20.00. The hardest part of the job will be reaching the lights. This fix also works for cracks in plastic tail light covers. Done with care the repair will be invisible and it will not fail in the future. For older vehicles it can be difficult to find exact replacement parts for things such as covers for tail lights, turn signals and clearance lights.
The type of plastic the lights are made from can be mended with a watery thin solvent glue made for bonding acrylic plastics. Weld-on 3 or 4 is one version, it can be obtained at hardware stores, Home Depot etc. A small can is about $16.00. To apply it get a very fine tipped artist brush, dip it into the solution leaving some of the liquid on the brush rather than blotting off all the excess. Lay the tip of the brush along the crack moving along the crack slowly but steadily and the liquid will wick right into the crack from the brush. Do NOT try to brush it on as you would when painting, this is a solvent and it works by dissolving the plastic so that it then fills the crack but if you get a lot of it on the surface that will make the surface cloudy looking. If a little too much gets on the surface do not try to rub it off, that will just leave a smeary looking surface. Do it with care and the mend will be invisible and watertight.
Another version of this type of adhesive is available from hobby shops and craft stores that have a section for plastic model making. Hobby Lobby is one such large craft store with that kind of section. The product you want to look for is Plaststruct Plastic Weld cement. This product is the same class of watery thin adhesive as the Weld-on it might come in an applicator with a needle like tip. But on a vertical surface I think using an artist brush gives you better control as the needle tip can release too much fluid.
Remember for this method to work you need a clear, watery thin solvent adhesive that can wick into the crack. Not just any old plastic adhesive will work.
I have myself invisibly mended cracks in plastic tail light covers and turn signal covers using this type of adhesive as well as mending an acrylic window on the front of the cab-over RV I used to own.
Now if the crack is mended and you still notice a small leak it might be coming from around the rubber seal that holds the lights in place. There is another thin adhesive on the market that can creep its way into and fill up those small voids. It is called Captain Tollyey's Creeping Crack cure. Yes it really does work but follow the instructions to get good results. It is sold at marine supply stores or online from Amazon and other websites. It is a go-to standby fix to have in your roadside repair kit for fixing pesky small leaks in window seals or the seals around other penetrations such as running lights. I have also had personal experience with this product, it does work for treating small voids where water leaks in.