I did alot of research on Ozone Generators and they seem to be the "industrial strength" method of odor removal.
They are as already mentioned, expensive, but they do work. You just have to be aware that they can also make one very sick while they are working so you dont want to be in that vehicle while it is running, if you decide to go that route.
Aside from that, just cleaning all the dirt out of the carpets and fabric if the seats are fabric, will make a huge difference in what else is left to smell bad.
This is where a good carpet extractor will really be good. I have not done a vehicle yet, even really clean ones, where I dont get a few gallons of dark brown water in the recovery tank after using the extractor on the carpeted areas, fabrics, and floor mats. It is still unbelieveable how much dirt is apparently down there at the bottom, that never comes out without help.
Next best thing would be a big compressor and a open nozzle to blow as much airflow through the vehicle, and in the process, moving alot of that dirt hopefully out the other side.
Have you removed the back seat/s to see if someone didnt leave something under there that melted, etc..?
Good luck with this.
DanF