It is where Fonzi said.

First unclip the plastic rivets and remove the cardboard cover under the dash. If you look up in there, you will see the ECU - Fairly large rectangular box. It has a spring clip that holds it in place along with three pins that just push into rubber grommets. Once clip is undone pull out horizontally at bottom and down. Can't recall for sure, but there may be a screw that needs to be removed to disconnect the harness plug. But there is enough slack to get at that. There is a second plug on end of harness that needs to be removed in order to remove the harness. There is a large grommet in the firewall which when removed, will just allow the ECU plug to pass through.

I rebuilt my harness last winter (with help from cushjbc on Benzworld) We learned a lot. These threads cover most of the subject:

D-Jet Engine Harness - Mercedes-Benz Forum

Winter/Spring Djet project - Mercedes-Benz Forum

We found a good source of the end rubber boots from a guy in Germany repro-parts.de. He also has the original type terminals and housings, but cushjbc introduced me to the modern double cantilever type used on Audis etc and I used those. The sleeve material from Wirecare works well and makes it easy to splice new wires to the main core of the existing harness.

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Graham
85 300D,72 350SL, 98 E320, Outback 2.5