A trick with rear speakers is to run them in series (8 ohms) and then run them on the bridged amp channel (which causes them to play mono). This will create a sense of a center channel speaker, while behind you, is better than nothing. It creates more of a "fill" noise than true stereo speakers.

And heck, if you are running short on speaker wire you might have to do it anyway.

As far as amps, I've got 2 old kidney style Rockford Fosgates. A punch 4040 and a punch 60ix. Great amps and you can find them cheap on ebay... if you can find them.