Check the radio remote receiver unit under the rear tailgate spoiler, the little box it sits in may need sealing with butyl cord sealant.
As well as renewing the receiver if it is full of water.

 

Many thanks for this. I seem to recall that getting this box out can be a real problem; is it easy or not? I cannot find a Haynes manual or similar anywhere that would help me.

 

Not too bad, be careful removing the trims and once the spoiler is undone, lift it gently a second person is needed really to hold it up while you disconnect the wiring plug and washer hose.