What I use to do with my car, and going to do with my truck, I bought a Bissel carpet shampoo/vac from lowes (a handheld one not like a vacuum size) and I use that with a good shampoo for it. Comes out looking brand new all the time when I had my car. I'm from NJ so we get salt and crap like that.
Another thing I did before I bought the Bissell was if I had a weekend where I can not use the car, I took the carpet completely out and power washed it and shampoo. But the Bissell was easier and did and better job and dried faster (that day instead of 2-3 with power washer).
I have also seen chemical guys sells a brush attachment for a drill and you just spray your interior cleaner of choice on carpet and use that attachment on drill and pick up all the gunk. They then did use a shampoo vacuum to suck the junk out as well. Really you could just use a brush then shampooer if anything.
Edit:
http://m.lowes.com/pd/BISSELL-SpotClean-ProHeat-0-289-Gallon-Carpet-Shampooer/50191473
Exactly what I got but mine is 3 years old so it's green. Yes if your carpets are bad which they may look good but when you use that you'll be like holy ****, it gets annoying because you'll have to empty/refill but I don't mind it to much, play some tunes and have some beers while doing it and your good!