Did you take the panels to the body shop for installation+paint, or did you install them yourself and just pay for the paint job?
Also, did you have the whole car repainted or just the "new" fenders?
My 240D is rusting in about the same place: a bubble underneath the paint about two inches wide and one inch tall, underneath the trim immediately behind the front wheel well, on both sides. The driver's side is a lot worse than the passenger's side...
'83 240D, 252k miles