I was getting no sound out of the system in my recently acquired 04 X5 and my cursory clean-up of connections, fuse checking, etc. yielded no results. I had no way to know if I had a bad amp or a bad "radio" unit. What I DID have, was a great buddy with a PhD in electrical engineering who was curious enough to help me figure it out.
He rigged up an external amplifier to the X5's radio and got sound to an external speaker - thus confirming the radio was sending a signal to the amp. He then figured out which wires were speaker outputs from the amp plug and bypassed the amp producing sound from the X5's speakers.
I was happy just to know for sure that the amp was my problem, but he suggested we keep going and see if we could figure out the precise problem. The only visible issue had been corrosion at the plug. I had cleaned that as best I could on both the inside and the outside, but you could see there was corrosion between the plug and the pcb (printed circuit board). He desoldered all the connections holding on the plug and removed it. There was indeed a lot of nasty corrosion in there and he brushed it all off thoroughly using a stiff plastic brush. Then he could see the problem.
On one of the traces, the corrosion had eaten away the connection for two of the pins rendering them disconnected from the pcb. The ideal fix would have been a new pcb, but he applied a cheater fix by running a connector wire on the other side of the board to connect to the other pins that went to the same trace. With that connection repaired, the amp was back to working. He said it would be likely that this same trace would be compromised on others' amps in the same way. So, if your amp is dead, you could try simply soldering in a jumper wire as shown in the picture below. If that works, you could then go to the effort of desoldering the plug so you could clean out all the corrosion between the plug and the pcb.
It's a fair amount of soldering work, but the alternative is to buy a used amp that may soon develop the same issue. Anyway, just thought I'd share.
AM.