Hell I used to take really bad carpet throw it in the bed of the truck then run it down to the car wash.
Attack it with some carpet cleaner, scrub, let it stand a while, then blast it with the car wash pressure wand. Works pretty damn good. The only rough part is the time I did the carpet in my '98 crew cab; getting that big piece back up into the bed of that tall truck was not fun once it was soaking wet and HEAVY.
Get it home and drape it across some barrels, saw horses, etc. or your hood if your paint was already toast like that truck's was.. and let it drip dry.
Richard