I doubt that you will be able to select just one rear speaker, probably a pair or none, therefore you could use this device,
http://www.mcmelectronics.com/product/AUDIOPLEX-TECHNOLOGY-MONOMIXER-200B-/50-6693
to send both L & R into a single speaker. Also, you have the right idea to direct the speaker toward the ceiling. This should give you a better effect than facing forward.
http://www.mcmelectronics.com/product/AUDIOPLEX-TECHNOLOGY-MONOMIXER-200B-/50-6693
to send both L & R into a single speaker. Also, you have the right idea to direct the speaker toward the ceiling. This should give you a better effect than facing forward.