Hi Baker31 and Welcome to BPO!
I have an almost 3-year old Greater Swiss Mountain Dog and we went through this little spell about 9 - 12 months ago. Ranger had NO problem getting in the car and then all of a sudden he boycotted.
![Roll Eyes ::)](https://bigpawsonly.com/Smileys/default/rolleyes.gif)
Ranger's command for getting in the car is "Load Up." So, what I would do is say, "Load Up" then get into the backseat where he rides with a cookie or toy. Like Steffanie, I do the "follow me" lifestyle. Since I got in the car first, he followed me right in. I'd praise him, give him the treat, and we didn't have any problems. Did that for about 2-3 months. Then I started giving him the command without climbing in the backseat. At first he balked, so I put his front paws in the car, and then he would pull his hindquarters up. This was followed by lots of praise and a treat. That was about another 2 months. Then, I gave him the command, a little nudge in the butt and he jumps right in. That is usually still what I do now. Sometimes he jumps right in, other times, he needs a little booty nudge.
![Cheesy :D](https://bigpawsonly.com/Smileys/default/cheesy.gif)
Lots of stuff can happen in a car. There are lots of loud noises (motorcycles, for example - Ranger HATES them), quick breaking can cause them to lose footing, etc, etc. The trick is to let them know not to be scared of that stuff, riding in the car is a blast, and usually leads them to somewhere fun.
Good Luck!!