My girlfriend's mini cooper amp recently died after being stored for a while. Basically the head unit would power on and act normal, but nothing would come out of the speakers. I heard people also having the amp cut in and out everyonce in awhile. After some research I found this hardon-kardon amp costs $1050usd new and $500usd used. Obviously replacing the whole head unit and bypassing the amp would cost less than either solution. Being the hacker type I figured I would try to fix it myself. After fighting it for a few days I finally found the issue. It was in the 1.2v switch mode power supply. The switcher had died and it was not supplying power to the SHARC DSP's core. The part was ST's L5973D, digikey part number 497-4566-1-ND. This is what died in her amp, your's may be different.
Replacing this part should be done with someone will a lot of surface mount experience. The die pad is connected to the ground rail and requires a smd heat gun to remove and replace.
Here are some specifics:
After reading forums I found that her amp of her 2006 Mini Cooper Convertable was located under the passenger seat. With the car off, remove the seat with a Torx socket. Then remove the 4 screws holding the amp case into the cavity. From there pull out the amp case, then remove the torx screws holding the top to the basin. Next remove the amp from the case by unbolting the 3 10mm nuts. At this point I would put all the screws back into the basin, and the car since you will probably need a bit to properly diagnois and repair the amp. Also, keep the amp cable out but put the seat back in and re-attach all the cables (BEFORE YOU TURN THE CAR ON!) If you do not do this you will need someone with an OBD connector to reset the airbag warning.
Now that you have the amp out you can remove the electronics by taking unscrewing the connector side, and the amp circuit should slide out. You should see the the two 470uh inductors. 1 is a 3.3v and the other is 1.2v. If the 3.3 volt is bad it will make the 1.2 volt bad since it feeds it. You should be able to hook up the amp in the car and test that this switching is working properly. You also can attach 12v to either side of the diode on the connector board in your lab. If you don't see 1.2v and or 3.3 volts you know where your problem lies. On my circuit I was not seeing the 1.2 switcher pulsing.
Hope this helps out the engineers out there locate their problems with this troublesome amps.
Also, if you want to connect your ipod to your mini-cooper stereo, this thread has all the correct info. You need to add caps to decouple the audio signal and a resistor to load down the amp a bit so it senses there is an aux connection available. An audio isolation transformer is not necessary.