Mix a cup of liquid laundry detergent (Tide) with a 3/4 to a gallon of hot water, manually mix it until very sudsy and use a stiff nylon brush to scrub the seats and carpets, follow directly behind the scrubbing with a shop vac. To dry it, park the vehicle with all windows slightly open and the heat on full tilt for about a half hour. I have used this method for years and even my 86' looks brand new.
Very dirty vehicles will require you to change the water often, I never recommend the use of harsh chemicals on any surface you are sitting or in an enclosed space like a vehicle.