Thank you both,
Sounds like a good start would be use my existing sub and amp and add a BM unit - whatever I find cheap on ebay
I've ordered a cable I need to run the amp from a speaker level input signal. I take it the bmw connector on the existing amp will have the connections I need? - can I "T" into them so that I don't loose signal to the rear speakers i.e. the L and R, + and - signal outputs feed the existing rear speakers and the 4 inputs for the amp hi lev inputs? I will also need a good earth point and switched live plus run my power cable from the battery - sure I've seen this covered in other posts.
Anything else I've missed?
Thanks again
Gordon