just wondering if you got into some sort of chemical spill during bush hogging, and nothing was ever reported, or was reported, and you just do not know about. and chemical is all caught up in the cab and HVAC ducts.
hyd oil leaks, antrifreeze leaks, other fluid leaks into the cab, been there done that, i just do not remember side effects, before i realized what happened, and fixed situation.
mice and other varmints are a pain in rump that you never able to solve. they get into wide range of areas, and chew stuff up. and some birds do the same thing.
double check exhaust pipe, see if you can "turn" the pipe. so when driving into the wind and with the wind, the exhaust does not come back into air intakes. easier said than done. have had my fair share of exhaust *ughs* i have had to adjust the exhast pipes a few times over the years, bolt/nut came loose, or vibration helped turn it some, to just hitting something.
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get some bungie cords, and strap the doors open, and open up the windows if you can, and see what happens. for a quick move of tractor.
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the one things i do hat about tractor cabs, it is not always easy to clean them out, its not like a truck or car, were you can stand on ground and crawl down to the floor boards and around seats. and everything tends to be "molded" so you need to deal with clips and screws and other fasteners, to get into the niddy griddy of things.
with above said, just bleaching or using some sort of mist or cleaner down ducts may only be a temporary solution. and if mice droppings or molds are the problem, till you remove the physical source of mice droppings or like from system, you will be back at a science experiment, trying other solutions as molds and things grow back.
