The pan inside in the back at the top inside has rusted through.
There are two thumbscrews, one on either side, remove those and it comes out, you might have to remove the screws that hold the thermostat in place.
The fan unplugs.
knock the rust loose and spray the inside with flexseal, or cover the bottom in flextape
reassemble
you can replace the door seal as well if it needs it, one can be found for ~$20 on amazon