See when doing the retrofit with the OEM parts it tends to be extremely expensive but you are guaranteed that it will work with your car, then again you need the correct stuff, coding, some wiring changes which could lead to alot of time being consumed and is the reason why the value is so high.
I did retrofits to my car from a good person and I can tell you it is a lot of work. Thanks to him I have a beter car but then again, I had most of the stuff when I bought the car.
I retrofit FRM3 and Bluetooth to mine, cause I wanted welcome lights :tiptoe: and I wanted to use the Navigation display to its fullest (reason why I fitted bluetooth to see my contact in that little screen on the dash) :rollsmile:
And it cost me R4K for just those 2 things. Be prepared to pay for it but it is worth every penny. Oh and I am the only one, that I know of, with welcome lights on a 2005 E90