Battery acid Anymore, my routine is with fabric or vinyl covers, remove the seat cover, take it to a Laundromat, wash on gentle cycle, air dry, (DO NOT PUT IN DRYER), ( you can't wash a leather cover) resew cover, glue a thin plastic, like cushion ease or trash can liner on infected area of seat foam, so acid can't wick back up into seat cover, and reinstall seat cover. Like I said, the cars belong to dealers, so I won't see them again. I wish I knew if this is the answer. I feel I have done everything I can do. I don't know what else there is to do. Thanks for the feedback, at least I know I'm not the only one with this problem.