If you open up the sides of the trunk like Hellboy suggested, you will see the problem.

The drain hose goes through a plastic connector that pinches it so that it can't help getting clogged. The bottom of the hose is smaller than the top!

Unfortunately, the drain ends inside the rear fender and you cannot get at the bottom if it from outside the car.

I have a piece of stiff wire, cut to length, hanging in the garage. It is not quite as stiff as coat hangar wire and is plastic coated so as not to damage the drain hose. It is actually a piece of high temperature electrical wire which has stiff insulation.

Every few weeks I pour a small cup of water down each drain, if it drains properly, I'm good to go, if it does not, I run the wire down the drain to clear it. Note - the drain takes a bend almost as soon as it starts, once you are past this curve, the wire slips in easilly.

As for my sunroofs, I clean them as much as I can and when one acts a bit gritty, I ask my dealer to have it lubed. Since we have 4 bricks serviced at the same dealer, they do it for free. (I know, I pay!)
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96 855R, 95 855,854, 90 744 Ti - 310,000 miles driven