Having now just learned that the G29 is very similar in set up to the E85, I am going to make another assumption, which may turn out to be wrong, but worth giving you a heads up just in case.
On the E85, the plastic trim under each side of the roof has the first couple of inches of the drain. This then connects to the main drain pipe which runs down in front of the rear wheel. If you rod it through too vigorously you can push the two halves of the pipe apart. The result is that the top looks dry and no water can be seen running out the bottom. It is all slowly filling up the sills!! So, just in case the G29 is dsimilar, take care when clearing the drain from the top.