I have been staring at this rear suspension on my bench at home for a while and finally decided to really take a strong look at the bump steer. I know there is a lot in the suspension once you lower the car or bounce a lot. Toe in goes crazy as the suspension moves past the center line of the arc. I was looking at some simple suspension geometry and even with the 5 link the rules basically apply. I made a quick tie rod by cutting an old one in half and extending its length by 6.5 inches to start with. Almost got rid of the bs completely. I turned the subframe up on end so it would be as though the car was sitting on the taillights and put a level on the wheel flange. You can measure toe in and out as the suspension travels just the same as you would camber roll. Way easier than using dial indicators or tape measure etc.
You can not get rid of all of it because of the caster roll[anti/dive]. The outside tie rod location changes its geometry to the upper and lower control arms. But it is minimal. Also I want less of this roll anyways so I hope I can make it even better. I will post up some pictures when I get something a little nicer made up. I am still thinking about how to make them for real and the mounts.

Have a look on 190rev.com , building a 190 sportline / AMG / something similar. They used a Faro arm to measure front and rear suspension points. It is a huge thread with lots of pics.

I have seen that thread. Unfortunately I do not see that he really made any changes to the rear end. It is a really complicated issue. I have made several changes to see the differences from stock to stock tie rod and adjusted camber and caster to adjusted toe with stock camber caster etc. All create different sweat spots. Actually the stock suspension is pretty darn good until it reaches near full compression. It is just the camber and camber roll from a lowered car that is bad. But once you adjust camber with an adjustable upper arm then things start getting wacky pretty quick. I am going to split the difference on the two tie rod lengths and see what happens. With the really long arm I am getting toe out on full compression but near no bump steer any place else.

Toe out at full compression will lead to snap oversteer. ( Tow out will steer the rear ) However, you might not reach full compression with stiff springs.

If you remove the cradle to body mounts you can lower the car about 1 inch right away.

Somewhere on this site there are pics of your multi link suspension that had a factory adjustable link ( one of the upper ones had an eccentric on the cradle side.)

Also, looking at my spare cradle from a 90 SL500, the upper link tabs are taller than they need to be making me think that some cars had subtle differences in geometry.

Well, I went back to the measuring tape to get a real idea of what is going on. I measured stock and with longer upper arms to simulate the changes I have made with my adjustable arms and stock tie rod and then two different tie rod lengths. The really long tie rod as I described before works best. Minimal total toe change and progressive through the suspension travel. The stock tie rod ends up with basically two spots that have a lot of change. One in compression and one in rebound. Right through the middle it is static. So basically on a stock ride height car it is near perfect as is.
But if you are lowered a little then it has around 9mm of bump steer on compression in a very short amount of travel. 3mm on rebound.
I think that some tweaking on the tie rod length and angle will get me pretty close to 0 bump and combined with stiffer springs and shocks I should be really stable.