I believe the headliner stays in place, the dome lamp is just removed and the sunroof motor, kind of stretching the ol' memory bank here, but between removing the motor and those access plates you replaced, I think that gets the job done. Yeah I've seen this too and I believe that replacing the cables will fix it up, make sure to lube the track to keep this from happening again. The cables get weak from popping out of the guide, then it has a "memory" to do this again, will pop out again without a problem.
I don't think warming the engine is any real great benefit to keep the plugs from breaking. If it's more comfortable to do the job you can be slower and more careful, there is probably an advantage to that which will keep you from busting plugs. Main problem for DIY is to keep from breaking the plug wire ends, MB has a wrench they advise to use for prying off the plug wires, research that tool.

Gilly

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