I know some of the earlier models there was a TSB but not to much info as to a permanent fix as SHOdded stated above. Ford motor company Explorer Exhaust Odor in Vehicle 12-12-4 2011-2013 Explorer
Some 2011-2013 Explorer vehicles may exhibit an exhaust odor in the vehicle with the auxiliary climate control system on.
Customers may indicate the odor smells like sulfur. This Technical Service Bulletin contains steps to correct the condition,
including replacing the left side rear air extractor, installing a new drain valve, and applying new layers of undercoating. BTW more INFO can certainly be found here. Z https://consumerist.com/2016/07/08/nhtsa-investigating-ford-explorers-over-concerns-exhaust-could-enter-cabin/