Kinda smells like something dead, doesn't it? You should be able to get rid of it by spraying Lysol or something like that in your fresh air vents. There is a TSB on this too that calls for special Mopar spray - which I've heard works well. I had this problem a couple of years ago. It ended up going away on it's own, and it hasn't come back. I don't remember the details, but I recall reading on here that it has something to do with the A/C compressor running with the defroster, and if you make a point of shutting down the heat before you shut down the engine, it helps keep it from happening. I've followed that practice for a couple of years and haven't had the problem since. I don't know if that's what's cleared it up for me or not. I'm sure someone else will chime in with a better memory than me.
If all else fails, make sure one of your "buddies" didn't stick a dead fish under your seat. :-laf