1998 BMW 328i HK stereo problem: Location of all components?

Hi, All,

I own a 1998 BMW 328i with the Harmon Kardon factory stereo upgrade.

This system has a switch on the console that turns on and off some kind
of spatial processing which boosts the bass and alters the "stereo
image" specifically for this model of car.

Recently I have had a problem where just the bass-boost and image
"processing" switches on and off intermittently, as if I was flipping
the switch. The system otherwise works fine, and the processing sounds
normal when it's working, so I know the problem is not a fuse or blown
component. I suspect a loose connector, but I don't know the location
of all of the components of the system.

I know about the in-dash head unit, of course, and the switch, but are
there separate amplifiers? Is there some kind of processing unit
separate from the head unit? Where would the various components be
located?

Thanks for any info or hints!

Ken