What is the ruling on writing "Remember Allah" or "There is no god but Allah" or similar phrases on the back of a car with the intention of reminding others?

It is permissible to write the name of Allah or any other ذكر (remembrance/invocation) in a place that is not trodden upon and does not lead to the degradation of what is written. Writing non-Qur'anic forms of ذكر is not disliked if it is not trodden upon; otherwise, it is strongly disliked, and treading upon it is forbidden.