I have a octavia estate 1.9 tdi plugged computer  in said faulty reverse buzzer when I put it in reverse car does not appear on radio but does briefly when I take it out of reverse all the sensors work OK was wondering were it is and how to do it ie remove trim and that

The OEM control unit is behind the trim on the passenger side of the boot, where you access the rear light cluster, It doesn't put anything on the radio display, just the beeper sounds on the Passenger side at the bottom of the " B " post