Lots of rust prone areas on the 123:

1) Rocker panels, particularly around the forward jack points, but also internally
2) Under all the side trim, particularly at the bottom of the doors, at the rear, where the stud passes through
3) Bottom of the doors, in the channel for the weatherstrip
4) Bottoms of front fenders, front (under bumper) and rear (especially under thr small molding at the bottom)
5) The hood, under the front and in the seam along the sides
6) Under the windshield and rear window weatherstrips
7) The rear floors, near the outboard side
8) The rear quarter panels, in the seam between the quarter panel and the outer wheel well (along the wheel arch), and at the lower rear

In addition to the areas mentioned in the previous posts. It is a shame, but the 123 (and earlier cars) area prone to rust. And they WERE dipped, injected and swathed with protective wax at the factory.