My sun roof decided to leak on me today (literally). When I got home and the rain stopped, I rolled the sunroof back and saw standing water in the grooves. Figuring the drains were blocked, I took a look and couldn't see anything. Scratching my head as to what to do, I decided to snake something down through the drain until it cleared the clog. Several tools were attempted without success, but a heavy duty mountain bike brake cable worked great- the right combination of flexibility and stiffness.
Hopefully the raindrops will no longer be falling on my head!