"Mostly" is an adverb and "so" is a conjunction. "Mostly so" does not mean "mostly because." We can't use "mostly because" in this sentence because it would create the opposite meaning ("mostly because he wouldn't stink up the car" sounds like this is something he would NOT do, but actually the writer is saying "so that" he won't stink up the car). "Mostly because" is used to explain the reason for something; "mostly so" is used to explain the result we desire. Examples: She goes to the fitness studio mostly because she doesn't want to get fat. She goes to the fitness studio mostly so she can be healthier.