The diagrams came out a bit bigger than I was expecting....
I installed a Blaupunkt THA275 in my previous beemer. Although it did have line level inputs, as far as I recall, the wiring loom that connected it to the car just took the standard dash HU speaker outputs for its input. (Hence my attempted diagram).
I don't think the standard head unit has line level outputs, or does it?
(PS. the THA is available for sale with a loom).
E90 325DSE manual - The model number on the boot may not be JUST the model number on the boot