From what you wrote, it maybe as simple as you getting the + and - wrong when you rewired the speakers, or it may not.

Rear speakers volume do not necessary have to be noticeable at all times. Some movies have a '5.1' soundtrack, but may not have many effects coming from the rear, nor do they necessary have to be loud.

If you're really fussy about this, then get a Sound Level Meter to measure it. Put your receiver into one of the test modes, which sends out white noise through the front and back speakers. You can then measure that all the output volume are the same, or make adjustments to make them the same.

I'm not sure what you mean by "until the rear speakers worked on stereo mode". When your amp is in stereo mode, the rear and centre speakers are not used.