The roof should still work even if they are up, try it.
There has a development. One of my other (Polish) mechanics came round to chat with the auto electrician who has been trying (unsuccessfully) to fix the demist grid on the back window, and we asked him "OK, how would YOU raise the rear passenger head restraints eh?"
So he clambered in the back, took a firm hold and physically pulled them up. And laughed.
Anyway, shortly after I needed to get access to part of the roof lining around the rear window that had become unglued and was allowing a bit to stick out of the side of the window when it's up, a very common issue.
And as soon as the lever was pushed CLUNK! both headrests fell back down.
I have read that you shouldn't operate the roof with the restraints up, I didn't realise that there is a back up system which drops them, and neither did the Polish guy, he was of the same opinion as the poster above, "Why not just keep them raised?".
So now we kind of know. I think you genuinely have to manhandle them up, but feck me, they're stiff! I mean, so stiff that I thought I was forcing the mechanism and might break something.
Maybe the ratchet is super stiff, it seems unnecessarily hard, and in the confines of a cabriolet with the roof up, particularly a car with so many features (inflatable chair pockets!) a real inconvenience. A woman driving with children would struggle.
Dunno, maybe it gets easier the more times you force them up?
Until we find out if they do indeed pop up with someone sat in the back with the seatbelt on, maybe, if the car detects it's driving over 20mph? (I'm just guessing) this important safety feature (according to Mercedes) seems like an afterthought.
Anyway I finally got the car back last night, and now, whilst it's sunny in Newbury... I'm going to put it's cover on.
I have to work and there's a bloody sycamore tree 200 yards away that dumps loads of its seeds on anything that's stationary for more than an hour.