I have a new to me 2005 MCS. On the way to work the rear speakers crackled and stopped working.
Question to the forum... Where would you start to diagnose the problem?

 

Is the car equipped with the Harman Kardon upgraded stereo? If so it might be the H/K amplifier which has let down a number of owners.

 

Yes I have the HK unit. I recently added the aux input, but the speakers worked after that install. Is the amp covered by the warranty?

 

The H/K amp should be covered if you're still in Warranty (four years OK, what about less than 50K miles?)

You should tell them you suspect the H/K amp, see if they've had experience. If not you'll need to visit them on a cold day when the amp is playing up so you can demonstrate.