You can have the ETA rebuilt with a 2-year warranty for about $275 through Don Roden Electronics in AL. The wiring harness can be purchased for around $500-600 and it doesn't take more than about an hour or two to install with no special tools. If you are planning to keep the car and the wiring harness is original, I would definitely change that. Usually the ETA's with ASR are more problematic. I don't know if your car had that option. My old 93 400E did not, but the harness for the ETA was still toast.
By the way, I accidentally discovered my ETA cable was bad when I was looking under the hood with the ignition on and the engine off. I was trying to figure out why my cruise control had stopped working. I was leaning on top of the air filter housing and it pressed down onto the bad ETA cable and caused the ETA motor to actuate. I discovered that every time I pushed on the housing the electric motor would run. Eventually I traced it back to the ETA cable underneath. Then I realized that I could get my cruise control to work, or stop working, by moving the cable in different positions.
The ETA is not hard to remove. It sits right under the mass air flow sensor or LH (hot wire) as Mercedes calls it. You will likely have to replace the hardened hose that goes from the back of the passenger side valve cover across the engine to the ETA. It comes in sections and it is difficult to replace just one section without breaking the next. You will see what I am talking about once you get the air filter housing off. The hose is #29 in the diagram.
http://www.w124-zone.com/downloads/MB%20CD/W124/w124CD1/Program/Engine/119/30-1262E.pdf