To test the EHA, you're testing mAmps. You first need the engine at operating temperature. Then, you turn off the engine and unplug the EHA. Then you add your multimeter (set to mAmps) to the circuit. You restart the engine and check the multimeter value(s).
In my picture, you can see my yellow alligator clips run from the front EHA prong to the front connector on the wiring harness. Then you'll see the red alligator clip runs from the rear EHA prong (the prongs are numbered 1 and 2, but I can't remember which is which) to the multimeter. And the black alligator clip runs from the multimeter to the rear connector of the wiring harness. So, the EHA's front connection is normal, you add the multimeter to the second connection.
You'll need to verify what your value should be. I expect it will fluctuate just above and below 0. In my car, I think it's supposed to be 0.3 to -0.3.
There are great videos on YouTube by FixMyMercedes and MMWA on your engine. You'll want to watch them. MMWA has one showing the set up I gave you here. FixMyMercedes goes through all sorts of diagnosis techniques and repairs.
EDIT: It's FixYourMercedes