Chiming in on the battery possibility. Remember that car batteries are filled w/sulphuric acid. And H2S gas (rotten egg gas) can be emitted when something goes wrong with either the battery or the charging system. The rotten egg gas could easily get drawn into the air vents under the windshield and go directly into the passenger compartment. It’s not possible to tell without a visual inspection, and the cat remains a likely culprit, but OP should at least consider the battery as a potential cause. It will likely prove a lot less expensive to fix if it is the battery or charging system.