De-Smell
Since nobody has replied, I'll give you some info.
The first thing you might want to do is one step more than a carpet shampoo.
A bucket of hot water, a scrub brush, and a Wet/Dry Shop Vac. This should be done after the seats are pulled out since the fungus-mungus is probably under the seats growing on all the french fries and crumbs under there. You might even consider removing the center console to get to more carpet.
Dump lots of water into the carpet, scrub it in, and suck it up until the water is coming out clear. Suck the carpet until it is as dry as it can be and don't put floor mats back in for a while so the carpet can dry. Leaving the doors open in a warm garage will help dry things out after this if you have one. A fan or heater inside will also help with the windows or doors open. Otherwise you'll just need to take it for a long drive with the floor heat on high and a back window cracked to dry things out.
The front seats have four bolts. You need to run the power seats to the front position to get to the back bolts and vice versa. Then unplug the power connector and pull them out. The bottom of the back seat will come out by just pushing it backwards and pulling it up. Same thing to put it back in, push it back with your knee until it catches the hooks and push down.
If you go so far as taking out the carpet you might as well replace the whole thing. Put down some Dynamat, some new automotive pad and a brand new carpet and you will have a new car interior.
If you have fabric seats you can also wipe them down with a cloth & hot water and shop vac them, or have them shampooed.
