Saab park assist ( SPA )

I am wondering if any of you guys have ever had to change any of your park assist sensors and did you D.I.Y it, if so could you tell me how you went about it.
I am getting the park assistance contact service message appear on the SID every time i select reverse gear. I have tried cleaning the sensor's with warm soapy water but that hasn't worked so i am guessing that one ( or maybe more ) of the sensor's is faulty.
The park assist sensors ( 4 of them ) are fitted into the centre part of the black plastic strip on the rear bumper. I am wondering if i can remove the faulty sensor by just removing the black plastic strip from the rear bumper to gain access to the sensors or do i have to remove the rear bumper of the car to gain access.
Any help with this will be much appreciated.