What is the ruling on working at a car dealership in Algeria that deals with both Islamic and usurious banks to finance car purchases?

Your work in selling cars is permissible. However, if buyers come to you intending to purchase with an interest-based loan before the loan procedures begin, and they ask you for something that would help them with the loan (such as a price quote for the bank), it is not permissible for you to fulfill their request; for that would be assisting in sin. But if they come to you after they have completed the procedures for the interest-based loan, then there is no harm in selling to them, because the loan has already been concluded, and the impermissibility is attached to the borrower's liability, not to the money itself.