Incorrect position shown on Audi A6 sat nav?

Old style A6 Avant in built Audi sat nav shows wrong position when it is wet for a few days. When dry all OK. Put Duck tape over GPS antenna at base of radio aerial and worked well for 6 weeks until left out in 24 hrs+ of heavy rain. Put non setting silicon round base and problem cured for 2 mths. But now problem back again.
Is a new antenna required or just removal of existing and water proofing? How do I get access to GPS antenna? There is a circular top Marked GPS which appears to be removable - how does it come off?