Originally Posted by Brian-bmw

This is incorrect. There are three versions of the E38/E39/X5 audio system. There is the standard stereo, the HiFi system, and the Top HiFi (DSP) system. The standard stereo system was never available in the US market, but it was very common in other global markets. In the standard stereo system, the radio drives the speakers directly, and there is no separate amp in the car. Read the technical details in the WDS and study the wiring diagrams for the standard audio for more details.