The at-fault driver--and a driver who rear ends you is almost always considered to be "at fault,' since he or she had a responsibility to keep such safe following distance and speed that he or she could stop in time--has to pay for your repairs and also for your car rental while your car is in the shop, at least for a reasonable time.If they or their insurer do not pay within a reasonable amount of time--and unfortunately, there is no hard and fast time period for payment set in the law--or do not pay everything to which you believe you are entitled, your only recourse is to sue the driver (you sue the driver) for the money; that's the only way to move matters along.