I actually had a friend w/a Sheltie that did the same thing. What she did was open all the car doors, set with her other dog inside the car, played gave him treats, and just made a HUGE production over the dog that was in the car. They just let the Sheltie do her own thing while they were doing this. I think she said that went on for 2 days. When the Sheltie eventually got into the car, she basically just “threw a party” and made a HUGE production over her (she also got hot dogs as treats while they were doing this–but only when she got in the car). Eventually, they took short rides (to the dog park, someplace where she could just have a TON of fun). Eventually the dog got over it. As people motivated as pits seem to be, I bet you could tailor this to suit your dog too! Good luck!