[Clearancediver]

You need to run your VIN through a vin decoder to figure out what radio option was the original install.

That said... given the description you have provided it's likely you had the high ending DSP version...aka as the "Carver" high end system due to the complete replacement you describe.... amp and speakers.

Replacing head with a BMW business radio may not be an option at this point.

You can check to see what business radio was originally installed and the suggested replacement radio via realoem.com if you were to pursue a OEM head replacement.