rear sub never worked
I do not think the rear sub ever worked - well at least not on the early models. My 3 is a 2005 and other than feeling a bit of vibration, that is about all one seems to get. The real problem is I do not think anything is wrong - that is just the way it is.
As to a fix, I do not think there is one. Maybe someone with a newer year 3 or 4 can comment as to if anything seems to come from it. I might note that within the radio head unit where one sets base, treble etc, there is a setting for the sub-woofer, but it does not seem to do much.
Also with the high end radio, there are no speaker wires as you know them exiting out the radio; a fibre optic cable sends the sound signals to an amp located under the front seats. From that amp, there are conventional speaker wires, some even leading to the rear to what LR calls a sub woofer and the rest of us regard as a nice try.
In the low end head unit, pins 3, 4, 5 and 6 plus 13, 14, 15 and 16 carry speaker signal in normal wires; in the high end, those pins are blank in Connector C1354 feeding into the rear of the head unit. Instead the high end radio has a dual cable fibre optic plug in the rear feeding to the NAV and to the Harmon Kardon Logic 7, (or not), amplifier. I say "or not", as there are two models of the HK amp, one that handles perhaps 9 speakers and the Logic 7 model that feeds 14 speakers if you can find them.
Premium Audio - Harman/Kardon LOGIC7 Audio System Speaker List
Item Description
01 - Front RH bass speaker
02 - Front RH tweeter
03 - Front RH mid range speaker
04 - Rear RH mid/bass speaker
05 - Rear RH tweeter
06 - Rear RH surround speaker
07 - Rear LH surround speaker
08 - Sub-woofer
09 - Rear LH tweeter
10 - Rear LH mid/bass speaker
11 - Front LH mid range speaker
12 - Front LH bass speaker
13 - Front LH tweeter
14 - Front centre fill speaker
Incidentally, I regard my high end Logic 7 radio as like driving around with the Boston Pops in the back seat. There is no thumping for certain, but high end sounds such as snare drums, piccolo or cymbal, well you never hear that in a Chev or even an Escalade. I think it is harder to make high frequencies clear with no static than bass, as it hides the static.
Matt Bettis Cymbals - YouTube