I've checked some old posts, but couldn't find for sure whether the S (which has rear speaker grilles but no rear speakers) is wired for rear speakers.  The 'Swing' radio in the FL has the facility for front and rear, so I'd like to add rear speakers if not too difficult.  Also perhaps better quality speakers all round, as the standard fronts sound a bit tinny.  Can anyone help, please?

Yeah, I wouldn't waste money putting the OEM ones in. I'd stick some co-axials in the rears and components in the front. I wouldn't put components in the rear as the tweeter position is too close to your ears in the front.