You typically shouldn't have to remove the headliner get the drains working. Open the sunroof and you should see the drain holes in the front corners when looking down from above. Try feeding a length of thin, somewhat flexible wire down through the drains to clear up any blockage. Eventually you should be able to pour some water in the holes and see it dripping out under the body. Once that is happening, I like to use a bleach/water solution to break up any mold or mildew in the tubes, which is often what causes them to clog up. Just be wary of spilling any on your interior, and also if done on a concrete driveway you'll have streaking from the bleach.