sounds like you need the help of an auto sparky... never fear help is hear.
if your car dosent have rear speakers, it means the stock head unit only has two channels. this means you will have to invest in a new head unit. rip out the old one. put a new one in. you should be able to use all the original wires and only run new wire for the rear speakers. you will need to identify the old wires going to the old unit. a test light is best for this. it should have an earth, an accesory feed, a battery feed, possibly a park light feed and the four existing speaker wires, two for each speaker, avoid probing these with positive, they are usualy two similar colours. you can check by looking at the back of the speakers, which you should replace as well. next wire up the new head, dont chop off the old plug just branch out, make shore you solder, (but not before you check that it works ) and insulate well, hey presto you got a better sounding stereo.