In the area of the EGR valve there is also the secondary air check valve. This system sets its own codes and is unrelated.
The speedo cable has to go from where the EGR attaches, to the rear of the engine, and then across the back, and then most of the way up the left side of the motor. This will be at least 40 inches (I'm guessing). I push the thing through by hand and mark the length with a dab of paint then I spin it against the stop till it moves significantly through. There is nothing that can be hurt. If the blockage isn't there the cable will reach the other side of the manifold and just ride there (you will not break through - at least not for a few weeks [bg])
To verify the activity of EGR place a vacuum hose to the nipple on the valve and suck on it while the engine is running. If the passage is clear the the engine will run significantly rough close to dying. Check before and after the cleaning for a comparison.
The hole doesn't have to be totally blocked to cause the light to come on.
Bosch Master, ASE Master, L1
33 years MB technician