Not sure if this remedy will help with your problem but when my windshield rubber and wing vent rubber gets hard and grayish looking, I use hand cleaner with lanolin to clean and soften it. The lanolin is important. I use Go Jo and or Whisk creamy hand cleaner without the grit. Gob it on and leave it for a couple hours. The rubber absorbs it. I then smear on more and rub it with a fine 3M scouring pad. It brings it back to life.