It's been a while since I worked on any 560SEL rear suspensions but I'm pretty sure the standard factory package would have been SLS rear suspension. The rear strut bottoms are accessed from below the trailing arm where two bolts hold that end in place. The top of the struts are accessed after removing the rear seat bottom first, then the three 8 mm screws one on each side and one in the center of the rear seat back, pulling down the arm rest makes that center one easier to find and remove. Rear seat back is then lifted upwards off its hangers; the strut tops are located behind the black elongated pentagonal plastic covers on either side of the fire wall. Each strut has an SLS hose that needs to be disconnected first, it’s 17 mm if I recall, then there should be a doubled 17 mm nuts holding the strut shaft top in place with a rubber donut and a washer above and below the chassis sheet metal.

The SLS system may still be holding some pressure when you disconnect the hoses watch for spraying fluid. If the trailing arms are not supported when you take the nuts off the strut shaft the rear springs will still be exerting their pressure and will release when the strut comes free, so don’t have your brother in law under the trailing arm looking upwards when you get to that point.

So your vehicle should most likely have Self Leveling Suspension in the rear, it works on hydraulic fluid pressure, you’ll likely find fluid coming from somewhere as you disassemble the system. If your looking for a replacement strut at the bone yard you’d look for obvious signs of fluid leaking from the rod seals, oil on the body of the strut or possibly a large accumulation of oily grime on the strut body could indicate the strut has a very small leak that has been leaking over a long period of time. Good pressure in the system when you loosen the hose at the strut would be a positive sign. Just barely cracking the hose connection will usually have quit a spray at pretty high velocity that will last a good minute or more if the shock is not leaky. The rubberized protective bellows boot on the struts are not available separately so in your search either pick struts with good boots of grab a better boot off a less than perfect strut and swap it. The only other concern with a replacement strut is the lower end ball joint this should be tight with an intact rubber boot. Good luck!