Is it permissible to continue working as a taxi driver, where customers heading to forbidden places like bars might ride, especially given the impossibility of knowing their destination beforehand and the difficulty of refusing to transport or drop them off?

The default ruling is that it is not permissible to transport customers to places of disobedience (sins), based on the Almighty's saying: (And do not cooperate in sin and aggression). However, if the driver is unaware of the passenger's intention and is surprised when the passenger disembarks at a place of disobedience, then there is no blame on him. But if the driver becomes aware of the passenger's intention before arrival, such as knowing or predominantly assuming that the passenger intends to go to a forbidden place, then it is not permissible for him to transport them. In such a case, he should try to find a way to drop them off, unless doing so would result in significant harm, such as imprisonment, physical harm, or a fine that would be difficult for him to bear. In this specific situation, it is permissible for him to transport the passenger, and the sin would be on the passenger. However, if the driver does not know or does not predominantly assume that the passenger is going to a place of disobedience, then there is no blame on him for transporting them.