This is an issue for 124 chassis cars though not nearly as frequent as it appears it is in 210 chassis cars. As they do for 210 chassis repairs MB offers repair parts as both the upper spring perch by itself and the upper spring perch attached to a section of the inner wheel well/box frame sheet metal.

The repair is not that difficult to accomplish, basically after disconnecting the shock and removing the spring, the old perch is removed, the area around and behind it is cleaned of undercoating and rust (wire wheel does this great) then you need to decide if there is enough metal left for a solid reattachment of just the perch by itself or whether the perch and sheet metal repair part is required.

I've done the job two ways, scab the plate perch over the rusted section, welding around the edges and numerous through holes in between, seal it up with undercoat. As this was welded, the areas exposed within the engine compartment where cleaned primed and painted after the welding heat destroyed the original paint, and the area inside the chassis box frame was douched with Wurth's yellow waxoyl product to rust proof that area.

And I've done it ala' the 210 factory procedure with a slight modification. The 210 procedure uses adhesive and very large and strong pop rivets. I poke some additional holes in the sheet metal and completely cover both sides of the repair with two-part panel adhesive, squeezing it together and then installing the rivets once the panels as close and tight as they can be gotten. The extra holes allow the adhesive to squeeze out rather than being trapped and forming a pocket preventing the metal from coming close together. After installing smear the adhesive over the holes and around the edges to seal things up, add some undercoating spray (3M seam sealer paste and/or spray shutz) and put it back together.

I've got about 5 or 6 years and 30K on the welded repair and the glued repairs on other's cars of a 3+ years at probably 15K yearly both methods without either problems or evidence of potential failure.

The repair parts from MB, used to be about $50for the perch alone and $230 for the larger perch/panel combo, the first time I did this I cut an oversize section from a rust free chassis and then trimmed it reducing it to a single layer of sheet steel with the perch attached.