The most common type of failure for a quarter window seal is simply age. Over time, and repeated exposure to the sun and the elements, it will start to crack and become brittle. When this happens, you may see chunks missing, or you may find large cracks around the outside of it. With cracks or chunks missing, it may allow air or water into the passenger area of your vehicle. After a rain, you could open your door to find a puddle in the back seat. This can result in more damage to the interior of the vehicle and much higher costs than a seal alone would be. Or driving down the highway, it could cause a very annoying whistling sound. If the quarter windows on your vehicle are stationary, meaning they don't swing open at all, the quarter window seal could be partially responsible for holding in the glass. If this is the case, having an old, brittle seal could allow the glass to fall out while driving, which is definitely not a good thing to have happen. If your vehicle has a glass piece that swings out, or opens in some way, it's not as much of a safety issue to get the seal replaced, but it could cause the aforementioned water or air symptoms.
Quarter window seals are typically made of EPDM rubber, which is a blend of rubber and plastic. It's designed to be soft enough to deform its shape when pressure is applied, but also rigid enough so it retains its shape when pressure is removed. Another material they're made from is silicone; however silicone is typically only used for sunroof seals since they tend to receive more heat from direct sunlight. Some manufacturers produce a coating that can be applied directly to the quarter window seal that will resist UV radiation and increase its life.
Replacing a quarter window seal should be done by a qualified body shop, but if you choose to replace it yourself, you'll need the right tools and you'll need to be very cautious not to cause damage to the body of the car or the glass. We've provided some very basic steps below to replace your seal, and since every car or truck is different, some of these may not apply.