Help for Really Stiff Leather Seats

I assume it is pre 71... if so, there are a few leather conditioners available. Some are more like saddle soap. Others are exotic and have various oils and extracts. The key is absobtion, not cleaning or for extrior look. I would do a light cleaning with a conditioner cleaner like Pinnacle, and then do a deep conditioning with a waxier type and rub in. Let it set in the heat windows rolled up. Check in a few hours and repeat. It may take a few reapplies but, each time thr leather will be more pliable and softer feel.

     My grandpa was a leather worker for 50 years.  I watched him take a civil war leather powder flask that was stiff as a board and soften the leather.  Other than a few superficial cracks it softened up and was very pliable.  Unbelievable for a 150 year old item
     He used neatsfoot oil.  I have use it for baseball gloves, when I was a kid, as well as for my kids growing up.  It is oily, but that is what you want.  You need something to soak all the way through the leather "remoisturize the leather".  I use an old paint bush or foam brush to apply.  Multiple coats liberally work best.  It might take an application a week for a month.  Or, until it gets soft.  Once you get your leather softened.  You can use some saddle soap to remove excess surface oil.  Saddle soap is just that SOAP.  It is used to clean leather.  It might have some wax in it, but will not do much to dried out leather.
     Be patient, it will soften up.  Just don't stress leather while dry, it will crack.  Let us all know, how it comes out