Look for the source that may be causing the stench. Rotting bits of food are a primary cause of bad odour. Bit from food that you ate may have slipped between the seats. Ensure you have not left any food items like frozen seafood, fresh meat etc. in the car.
To remove any food particles or other odour causing materials that might have found a place in the fibres of the floor carpet, vacuuming the carpet thoroughly will do the trick. Take a handful of baking soda, spread it uniformly on the floor carpet, rub it into the carpet fibres with your hands and allow it to settle for a few hours. Then vacuum the carpet once more.
If you want to get rid of odours, charcoal is one of the best natural products. Place an open bag of charcoal in the car and leave it there for a couple of days at least. A glass full of white vinegar placed in the car’s sup holds helps as well. Leave it there for as long as you can but you should remove it before driving to avoid splashing. Citrus oil is another common item helpful in removing bad odours. Cotton balls dipped into citrus oil should be placed in a small open container.
Use coffee grounds as is common practice of most truck drivers in the onion transportation field. Hang a pouch full of fresh coffee grounds in the car. As it dries, it will absorb most of the stench and leave behind the scent of freshly brewed coffee which is not bad at all.
The odour might be caused by dirt filtering into the car due to an air filter that has an accumulation of dirt in it. Get the air filter replaced or if you can, do it yourself.