5.1, no rear sound

Ok working with my 5.1 system, having problems. The rear speakers take no signal. I have Windows set to 5.1, test sound (bells in each speaker) have no effect in the back. I switched the wire out of the back of my sub(logitech 5500. 3 plugs out of mobo, into control pod, into sub, from sub to 5 speakers). All the wires take a signal fine, and i can plug the rear set into the front set's plugs, and i hear the fronts' sounds fine. but if i plug the front speakers into the rear plugs, nothing.

Any ideas?

OP here. Friend narrowed it down to the internal workings of the sub connections that go to the speakers (loose screw or broken lead). That or an internal fuse, either are fixable.