Brilliant thread! I've just spent an hour cleaning out a couple of gallons of rain water from my spare wheel well and roof compartment, so now I know what I am facing.
One question, maybe a bit silly, but... where do these drain holes lead to? And more specifially, what does prevent us from putting the air hose at the bottom end of these drain holes and blow all crud out the other direction, leaving the roof structure where it is?
Or would that be over-simplifying things a bit??