Take a can of Tuff Stuff and lather that shit into the carpet.
Your gonna be there scrubbing for a while but the shit and grime and grease and dirt and if you smoke and spill some buds on the carpet it will pick that up also.

Its pretty useful on the interior roof panel.
Gets alot of dirt out also.
It cost me like 6 bucks at Pep Boys and you get two cans for that price.