I added a pair of 4 x 10" speakers below the rear seat (weekender) and
installed a small amp under the rear seat. I connected the new speakers and
the existing rear speakers to the front/ rear speaker outputs on the amp,
this in turn was connected to the RCA outputs on the deck, I left the
existing front speakers connected to regular front speaker outputs on the
deck. This gave me use of the fader and balance controls and it sounds way
better than stock, while not your rolling boom box bass it does sound solid.
I mounted my CD changer upright  between the driver seat and the rear facing
jump seat (not a Westy).

Derek
'87 Syncro