Lots and lots of elbow grease and a variety of good old fashioned cleaning solutions (carpet shampoo, upholstery cleaner, and so forth).
And I’m a little surprised that the dealer didn’t try that. An ionizer won’t do the trick. What’d they do, drive to Kragen and spend $2.99 for the thing? Holy cow.
Elbow grease should go a long way to fixing this, though. It really should. Don’t waste your time looking for a miracle product to fix this. They don’t exist.