I have a 2008 E93 with cream beige leather interior, which I've owned for a few months and only just started using in earnest with the roof down (thanks to our fantastically sunny English spring ).
The rear parcel shelf aft of the rear headrests (it's in textured beige plastic, with some parts just behind the headrests in smooth beige plastic that fold when the roof is retracting) gets dirty very quickly as it's effectively part of the outside of the car when the roof is open.
The cleaning kit that came with the car includes exterior plastic cleaner, and a plastic protection emulsion.
What do you E93 owners use to clean these parts? I hesitate to use normal household cleaner (e.g. Ecover cream cleaner) in case it's too aggressive, but I'm not sure soap and water is ideal either.