i had/have the same problem with Shasta. At first she was terrified of the cat because it made her sick. She hadn't had much experience with it. Now she enjoys rides but is unsure of how to go about getting into it. She really wants to. She whines and tries and she's getting a great deal better about it but still has trouble. I drive an Xterra so its pretty tall but she's even afraid of getting into our mustang and thats pretty low to the ground. It may be your pups size that is intimidating as they're not sure how to work so smoothly like adult dogs. They're clumsy and gangly. Shasta only recently discovered she can use the running board to get in the car. Only thing is i'm trying to teach her she can climb up onto the floor and THEN the seat but she insists on going straight to the seat. I encouraged her a few times a day for about a week before she finally started trying harder to get in on her own. BUT i also have to be standing between her and the door patting the seat encouraging her that she can do it. After she got over her initial issue getting in, i started treating for her being calm and relaxed. When she would just put her front feet up in the car, there would be a party. hope that gives you an idea. how old is your pup?
Shasta GSD 5/5/10 CGC, ITD, TC
"Dax" Thor z vom Weberhaus BCAT CAX2 CGCA ETD HCT NCO-1 PKD-T RATI RATN RA TC TKA 3/18/2013
"Hades" Guilty vom Blutfeuer 10/26/2018
Zena GSD 6/1/03-2/16/2016
Riley GSD/BC 1/10/05-2/1/2013