The tubes are located in each corner of the sunroof. The front two are relatively easy to access, the back two not so much. You'd best take off the roof panel per the FAQ instructions to get to them. Do you have a lot of leaves and such where you park the car? Most likely to have this problem if this is the case.

The cowling is the metal piece between the hood and the windshield. In the middle of it there is a metal mesh screen which with time falls below and may not even be visible. The opening fills with debris and this eventually plugs the heater motor intake and impedes its movement but rarely will cause leaks.

Another leak source is the footwell flap which may need to be resealed: this is covered well in the fAQ.