IT can be done! I had a Acura TSX without navi and many people and dealers say it could never be done as the existing harness, connectors are not the same.
Well, what happened?? One guy started building his own connectors, harness, and bought a service manual. He spent 2 weeks and posted his work on acurazine.com. Thats when 10 of us including me, did the same thing! we bought a insurance salvage/pulled navi system, radio, dvd drive, etc. and retrofitted it. Everything, wiring had to be custom made and hand wired and built! Took me 1 week!
for example, there were multiple wires you had to run throughout the car to key components in my Acura TSX. These include: VSS (vehicle speed sensor) to the ECU, Can-H, Can-L bus differential (to the dash gauge cluster), other ECU parts, A/C and HVAC tapping, and other stuff like ignition, ACC (accessory),etc. You just have to go find where they are located!
Looking at bimmers, the change will be more significant including completely replacing the dashboard plastics, custom building a harness cable , tens of connectors (just like the acura) and custom pins to tap into various parts of the electrical system.
Hence, two things will be needed! the complete bible service manual to the car and the electrical manual!
I believe it can be done! just need time to research and study the parts and money to buy the parts...
but i learnt my lesson! i won't go through that hassle again! so i made sure to GET the damn NAVI this time!