I have never changed the motor assembly but I have taken the headliner off the sunroof and lubed the cable in my 86 190e. Taking the motor out shouldn't be too daunting of a task, I thought I was going to have to do it and found the motor in the trunk. I think that it was only secured by about 3 - 5 bolts and had one electrical plug on it. Again, I'm not sure how similar it is to the 123 but the motor should be relatively easy to find once you remove any trunk plastic/lining on the drivers side.