Fade F to R not working on Sony CDXF7000

Question

I bought this truck (2002 Dodge) used with this radio already installed. The sound is balanced evenly between the front and rear speakers when the fade selection is at either the F (front) or the R (Rear) setting. In the middle of those two settings (0) I have hardly no sound. The balance left and right works properly. I checked the speaker wiring with the installation instructions and it appears to be installed correctly. I also tried re-setting the unit and that made no difference.