Time, time, and more time.
It will eventually go away with all the good suggestions on here, but it does take weeks/months to fade completely.
We forgot a Halloween pumpkin in the back of the quad cab pick-up. And it rotted/melted into the carpet. OMG - the stench was unbelievable. I thought I'd have to have a forensic cleaning service come in.... all b/c of a pumpkin! but after using the regular tips/tricks it just took time to air out.
I second using crumpled newspaper inside when it's not in use and also prop door open.