How do I clean out the smell of rotten watermelon from a car?

Natures Miracle. It’s live enzymes that eat organic matter.

They need to live long enough to do their job, so the car must be parked in a cool place and the NM must be applied so that the items are saturated and covered/wrapped with plastic wrap (like Glad or Saran Wrap) and left alone for at least 24 hours.

If you've already put cleaners on them that will kill the enzymes (like shampoo, windex, Amon is) then here’s the solution:

get a wet/dry shop vac (take out the filter & empty the dirt) and use it in wet-mode.

Fill up a spray bottle with distilled or at least non-chlorinated water. Thoroughly Spray down everything that’s been treated and then shop vac the water out. I’d repeat that until the old cleansing chemicals are rinsed out.

You can buy Room Shocker to remove odors.... https://www.youtube.com/watch?v=e-pT9uhp6MU

Have your car detailed. They do a great job.

If none of the other suggestions don't work. Have the carpet replaced by an auto upholstery shop.