What is the ruling on working as an intermediary between a car company and usurious banks regarding their financial dealings, such as depositing money and checks?

It is not permissible for the questioner to engage in any work related to usury, even if it is merely carrying or transferring a paper; because it involves cooperation in sin and aggression. Allah Almighty says: "And do not cooperate in sin and aggression. And fear Allah; indeed, Allah is severe in penalty." If the questioner is able to remain in this agency in work unrelated to usury, then there is no harm. Otherwise, it is not permissible for him to stay.