Aloha Jim,
I don't know how badly cracked the rubber is. I have had good luck with Armorall on some really nasty looking rubber and after a bit of time and several applications, it was acceptable unless it is a show truck. Mothers Back to Black is another good product id the color is gone from your rubber. Some guys don't like Armorall 'cause once you start using it, you can't stop. That said, I have been using it since the mid 70s on everything from Ferrari to Ford and have always been happy.
God bless Ed