I used a small diameter drill, moderately long, and rotated
it by hand *very* gently applying light pressure towards the
back of the hole.

I disconnected the blocked tube at the instrument panel and had a friend
blow into it while I used a drill as described by Todd. Takes only 5
minutes, a drill and a friend, no risk of popping the tube off, no debris in
the tube and no fiddling with vacuum. If you are worried about humidity you
could use a small fan for inflating water toys or something the like (don´t
use any kind of compressor!) for blowing.

Michael