Had problems with blocked roof drains and found little info on G01. So after much investigation found the following fix -
Remove rear side trims in boot
Remove side trims Nr boot cover release and same other side
Lower rear edge roof lining just pull off snap connectors
Locate black plastic drain tubes in both sides of roof they track along then down into rear light areas
Carefully pull and remove tube at connection point in roof - they’re tight but they do come off.
Test with water down tube if nothing appears under car they tube is blocked.
Use thin tube or wire to gently clean out drain tube then blow hard with mouth -not airline - to clear debris. You should now see water dropping from underside of car. Use small diameter plastic tube in upper drain connector in roof Feed tube into hole until it appears in the Roof tracks on top of vehicle.
Test both sides to ensure water passes freely through all drains by pouring water into roof tracks if all good reassemble.
Front drains are easier - use wire to carefully find drain hole in front of sunroof frame clean the hole and feed tube or wire carefully down hole. Again blow mouth only NO airline. Test and look for water exiting from Front door near hinge - the drain seal is very visible.
Finally use hose to drench test. Good luck. Don’t use air line and don’t use sharp wire !