You actually did get some advice on how to go about retrieving the body. And restoring the car.
Did you not see those replies ?
Did you appreciate the help?
If you get advice but only come back to ask a question whether a dead stinking animal body inside of a car will lower its value, well, people wonder.
Like I said before, its an unfortunate situation. Lesson learned.
The value of the car will definitely plummet unless you find and remove the dead body and any parts that liquids may have seeped into. Time if of essence. Your best bet is a garage that also does detailing. They will know how to remove seats and panels and carpets, to get to hard to reach areas. They usually also have experience in removal of filth and odors. The longer you wait, the worse it will get, though.
I'm not sure what else there is to say or do.
I wish you good luck, I'm sure your boyfriend is probably not thrilled, to say the least.