I bought a Hizoo single DIN stereo for my e39. Everything functions except no sound. A minor detail lol. The harness is a 40 pin connector round pin style. I believe my car has what they call the business radio setup, an upper unit was the cassette deck and lower had radio functions. It had a 12 pin connector. The amplifier is in the trunk. This car did not have Nav or DSP. I think the problem lies in that the amplifier is not being activated but I cannot figure out how everything ties together and the manufacturer seems to be of no help. Any ideas?