To the best of my knowledge, there were never any rear speaker options in the earlier 107's. I have a subwoofer in my 85 107 and a company called Precision Auto has a rear woofer set up that appears totally stock. If you want to maintain stock appearance I don't know another way. If you build boxes, the sky is the limit.

Steve
PS - Precision auto info nto handy but post if you need it.