The dog has probably been conditioned to stay in the trunk since it was a pup. That's my guess. I say this because my dog was 8 weeks and I used to contain it in an x pen in my home. As he grew he could have easily gotten out. I can use it today and he's I've 120lbs but he will stay behind it. He is agile enough to go over ut. He's strong enough to knock it down. But he still views it as a barrier. He would go through it if he thought were being attacked.
I don't have a pickup truck. I've thought about getting one to use with my dogs. But I would crate them when they're in the bed. One might ride shotgun.