Most of what you need to know is in this
link.
http://www.e90post.com/forums/showthread.php?t=641323
If you go to the above thread an look at entry 4 you will find this
CTuna's HiFi Install DIY w MusicarNW Kit
My E91 VP Kit install.pdf I know you said you have
a base system but it may be of some use.
The MusicarNW kits contain everything you need to install
the system.But they may be hard to get in Norway.