Left rear quarter panel

Two options after removing all the jute insulation:.

Use a compressor and a long reach blow off gun to blow it out, possibly blasting bits of dead rat and poison over the interior of your car.

Remove the side marker light and fill the area with water and use a wet-dry vac to suck up the bits that float to the surface. rinse and repeat
iirc, there is a drain plug down there. Also use a long reach scrub brush and a enzyme cleaner like natures miracle.
If your quarters are already rusted, you got nothing to lose.
If not, as a final step blow it out until dry with the compressed air (or shopvac blower if it has that option), hopefully nothing organic gets blown back.

A LED light and a inspection mirror should get you a good enough view to see if there is anything left.

Disclaimer: I've never had to clean animal remains from any of my vehicles, just dog diarrhea from an over excited Labrador and cat piss from a neighbors cat climbing in through open T-tops.

Thanks for the colorful, yet very explanatory, response. Fortunately, it appears I got most of the remains with my first effort. Closed up the garage all day yesterday and it didn't smell bad at all when I opened it up after work. I still need to disinfect the inside of that quarter panel, and I thought about removing that side marker. I know there are a couple of rubber hoses in there, not sure if they were connected to the drain or what. My priorities are now shifting to getting the rest of the interior out and cleaned up (lots of rat poo still in there) with a dry vacuum before I start getting anything wet. Once I've done that, then I will attempt the final cleanup… that particular quarter was, indeed, in need of some repairs, so you're right - I've got nothing to lose.