[Greasey Harley]

What I do:
Remove the seat, carpet/floor mat
open the kick panel vents
run a garden hose full blast in the cowl, pointing it down at the vent.
let it run, all kinds of crap will run out of the vent onto the floor.
repeat for the other side.
while you're in there, cram the hose down in the cab corners (behind the seat) and flush out all that muck too. (this will also flush the rockers out). Just let it run 'til it runs clear ...then let it run some more.
Let everything dry thoroughly before you reassemble. Do this in the summer and everything dries out fairly quickly.
The amount of crap that comes out the first time this is done to an old truck is incredible. This is one of the first things I do when I get an old truck.