This is a simple way to add a rear channel to any stereo system. In my pictures I show a third speaker wired up for testing, but the third speaker should be mounted in the rear. The rear speaker will play only what is different between the two channels. It is not very loud, but it is noticeable. The lower it's impedence the louder it will be. The speaker does not need to be as good of quality as your other speakers. Don't go spending a lot of money.
This is not my original idea, nor is it new. Brian Eno discovered this and did a writeup about it in the early 1980s.
Step 1: Hooking It Up.
Attach one of the two wires from the rear channel to the positive connection for the left channel and the other one to the positive connection for the right. It does not matter which wire from the rear speaker goes to which channel.