S60 rear door and parcel speakers

[Umberway]

Hi All,

Ahppy new owener of an S60 D5. Was ready to be blown away by the pro-logic stereo (HU-850), but sadly lacking - the rear NS door and parcel speakers aren't working.
Recently wired in a GROM IPhone adapter and all was OK behind the head unit.

So where to check next - I've read they are wired together, where is the amp, or at least a good place to start looking?

Ta.