I suspect there are very stinky mushrooms growing in the debris inside the ventilation system. We live in Florida and the van sits outside in an area in our yard where stinky mushrooms grow at certain times of the year. I have tried spraying Lysol in the outside vent with the fan on full. That seems to have helped ove time but can I dump some vinegar in there without harming anything? Any other suggestions would be appreciated.
First I would suggest cleaning out the drains and the catch areas under the grill in front of the windscreen. If you pour water in there, it should come out under the car, usually about where the passenger puts their feet. If it is clogged, a little compressed air (Just a little, or you can blow the drain hose off). You can also try using some string trimmer line. Again don't get too aggressive.