Well, if center would only work if htere was a surround mode enabled, otherwise it would only ever be front left and right. so if center is working, rear should work unless something is wrong with them.
Fact is, using RCA is never going to give real surround period. No way to pass 6 channel with only 2.
You need a AVR that has multiple HDMI inputs and an HDMI output that goes to your TV, so all your devices plug into the AVR from HDMI and then you'll get full true surround. Anything else is only going to be simulated anyways.