Is the part number for your head unit really "RCD150". Ive done a lot of reading about VW head units up until getting an RNS315 a few weeks back, and thats not a number I recognise. Unless you meant RCD510? In which case it does not have bluetooth built in (only the RNS315 does) and you will require a separate bluetooth module which connects up to the head unit and microphone.
I guess the reason it wont let you select BT mode is because there is no physical tranceiver anywhere at the moment.
A google search should quickly clarify this for you as there are plenty of threads out there about this. Try searching for "1k8 035 730 D". Not sure if thats the exact one you need, but thats the kind of thing.. youll have to do some research as I also believe there are different type which offer different levels of functionality.
