have you tried just leaving the harness laying where she can see it, smell it, pass by it but it not getting put on her? She may be uncomfortable with how it looks. I know it sounds strange but it does happen. Shelby does like the way slip chain but is perfectly fine with the prong. If she even see's our other dogs slip/choke chain, she gets twitchy, tail between the legs and runs for cover. We started leaving it out where she can see it and sniff at it and gradually she's getting better with it. She no longer runs when we pull it out but she's still twitchy and tail between the legs mode. We've never used a slip with her either so i honestly dont know if the slip was used abusively with her. I've also had cats who got freaked out by a crawling handle. Just like with people, we dont like seeing or looking at certain things because it makes us uncomfortable. Animals can suffer the same. she may not entirely associate the harness with leading to the fun of a car ride.