Blocked sunroof drain channels can cause water to enter the cabin, but it is important to be delicate when attempting to unblock them as it is easy to damage the channels which would then require a more expensive repair. You can check the performance of each drain using a small amount of water, and then use something flexible such as a pipe cleaner or coated garden wire inserted into the drain. You can also use compressed air to free any blockages if you have access to an air line.