I'm assuming that the 123 is fundamentally the same as the 126 - if not then the rest of this will not interest you!

The seat base is made up of a series of wire rods bent into springs, and in some places the springs are welded to each other. What will have happened is that several of these rods have broken, allowing the seat base to collapse.

If you have time on your hands, and are adept at welding, you could presumably take the seat apart and attempt to repair the spings by welding them back together again. The rubber/hair pad that rests on top of the springs, and over which the upholstery cover is stretched, will likely be past it's sell by date too, so it will need seeing to.

In the case of my drivers seat I decided there was no true repair, only a partial replace. I removed the seat from the car, dismantled the seat back from the base, removed the leather covers and separated the spring base from the motor assembly and rails.

I ordered a replacement (genuine MB) seat base spring and rubber/hair mat from performance products and put everything back together using the new parts.

I still have the old seat base and pad, and could probably find time to take pictures if anyone is interested in seeing what they look like. The new spring base has reinforcements in it that were not present in the original, perhaps it will last longer?

There are posts out there already that detail the whole procedure, and include pictures from memory.

Kevin

__________________
'85 300SD