as far as I know... the run of cable goes like this:
-Back of the gauge (with a barbed pressure fitting and zip-tie)
-Along the gunnel on the starboard side
- Into the alpha drive from inside the boat
- out of the drive at the front of the leg, just above the cavitation plate.
- into the pick up.
If you trim up your drive you should be able to see the rubber hose, simply pull it off the pick-up. Then remove the pipe from the back of the gauge and blow thru it (from the gauge to the pick-up end) with the compressor to clear an gunk.
I'd actually check the pick up first though as this is more likely to be where any crud has gotten in.
Also, when you remove the tube from the gauge, ensure it drops to zero instantly, if it stays at 30MPH, it's the gauge that is bad, not the pick-up/tube. As a simply (yet highly entertaining) way of testing the gauge do the following.
Remove the gauge from the dash, call a helper, and then get them to blow as hard as they can into the back of the gauge (as if they were blowing a baloon up). When they do this you should be able to see the gauge go up and down. 8)