Zipp didn't end up in the back seat. Behind the back seat is the trunk, in those older model cars they're not covered.

He climbed in because his Lickitung was sitting there, and he was looking for some comfort.

Only Lickitung was in the trunk, and then Zipp climbed in with him.

Anyway, I'm working on the next chapter right now, and I'll post it right away when it gets finished.