Assuming it is the linkage. The motor and linkage are usually combined into one compact unit, unlike front screen wipers. I suggest you buy a second/hand one from your nearest scrap yard. £10 is an average price. If you remove it carefully from a scrap car yourself this will give you practise before you remove your own car's motor. Ask the scrap yard to give you a week's guarantee to fit the motor and try it out in case it is faulty. Make sure you have a good earth connection to the rear door before going to the scrap yard.
