You should soften the rubber weatherstripping before you install it on your vehicle. If you don't, it will age quickly and become dry and cracked. When it does become dry and cracked, you will hear wind whistling through it as you drive and the rain will leak into your vehicle. You just need to condition the rubber before you install it and this problem will be prevented. It's an easy project you can do using supplies easily obtained from your local auto parts store.
Fill a bucket with water and a little dish soap. Use a soft sponge to wash the weatherstripping to remove any dirt.
Rinse the weather stripping with a wet towel. Allow it to dry.
Soften the weather stripping by spraying a clean towel with some silicone spray and wiping it on the weatherstripping. Wait for weatherstripping to absorb the silicone and then repeat the procedure.
