Hi, rodent damage to the wire harness is obvious upon inspection and is also easily repairable in most cases. I would have the dealer show you the damaged area and inquire about simply repairing the gnawed wiring. If they are unwilling to do this for you I would take the car to an independent BMW specialist to have them make the repair. I cannot imagine that the entire wiring harness needs to be replaced due to rodent damage. I repair rodent damaged harnesses regularly on vehicles with no adverse effects. It sounds to me as though the dealer is trying to sell you more than you need to get this repaired. Your BMW has electronic power steering so damage to the wiring harness will cause the steering to be stiff.
Please let me know if you have any questions.