The previous owner has installed a piece of wire clothes hanger as a substitute for the control rod that goes to the cruise control.

I suspect this might be part of the problem but I also notice that the lever on the engine side of the IP is missing a control rod.

It's the piece with a spring attached to it and also another control rod.

Am I missing a control rod?

I have looked it over and I don't see where a rod connected to the bare ball would connect to.

The bare ball is the lower one.

When I try to engage the cruise in it's current configuration I get a fraction of a second where it seems the cruise is engaging and then rpms fall away.

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1993 W124 300D
-297K on the clock as I type this.