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It is unlikely that the drain tubes are "clogged". They run at least 6 feet long and for the most part are in between two pieces of sheet metal, as well as being made of thick plastic. Even if there were stagnate water in them the smell would not make it to you. It is more likely that you have a small leak in the sun roof or the top of the windshield. This allows water to get in behind the headliner. Most headliners have a backer on them and so would not look or feel wet. However, this backer is coated in an adhesive that can tend to smell a bit ripe if wet. I would not recommend trying to remove the headliner yourself. It is very difficult to do without damage even for a professional. You should contact your local body shop and let them know the problem, it will probably run you about $80 to remove the headliner and another $150 or so to replace a sunroof weatherstrip.