No matter what Bentley says, my comfort relay is under the driver's side dash, next to the steering column. Maybe it's different on cars with right side steering. Once you know where it is, it's pretty easy to get to. Remove the trim panel under the steering wheel (3 screws). Underneath the trim panel, you'll find a metal panel (knee guard?) which has to be removed (3 recessed 10mm bolts). You'll need a socket wrench with an extension to get at these. On the door side of the steering column, there is a plastic holder which holds several relays, this can be unfastened and folded down. The comfort relay is light blue or gray, mine was blue.
Having said all this, this turned out not to be my problem. I had the same issue: windows & sunroof stopped at the same time. I never realized that my power door locks were also not working. I checked all the fuses which I thought had something to do with it, nothing. So, I replaced the comfort relay...still nothing. Once I realized the power locks were not working either, I checked the fuses for that. Turned out that fuse #7 (5 amp) had blown. Once I replaced this, everything was working again. Of course, I needlessly replaced a good comfort relay, but it was only a $25 part.
Who would think that a 5 amp fuse for the central locking system, could cause so many things to stop working. Only some devious German engineer could have dreamed up that wiring scheme.