Jim, Your memory is just fine; the ignition switch removes pretty much as you described. Even with the sides of the box off, I had to take out the ignition switch to get it to clear. I couldn't find many threads about W124 heater core failures so I am not planning to replace mine. Did you replace yours? What did you do about the disintegrated foam around the heater core? I am wondering if there is a possibility of the core rattling in the box. Thus far, I have spent about 12 hours over 5 days and am going to resume Monday. In the meantime, I occasionally walk out to the garage and gaze at that heater box contraption and pile of interior parts laying on the floor.
Frank