Confused about HU install

Here we go, i just bought 99 trooper w/out premium sound. It had an aftermarket $30 cd player. So naturally I wanted to switch it out for a sony CDXF7710. Whoever installed the previous unit didn't use a plug in factory wiring adaptor. They spliced into the existing harness for all connections including all four speakers. All speakers worked fine. I cut wires going to the old HU & planned on using all the same connections.

For some reason I can only get sound out of the left channels. Of course I checked the balance & other settings on the HU. I cut all the speaker wires except the left front,(I figured the right channels of the amp section were bad on the HU).  Then connected the left rear channel out of the HU to the right front speaker & it worked. I exchanged the HU but still have the same problem. I can't imagine what the wiring problem could be, i matched up all speaker connections with the ones from the old HU.  I have been installing off & on for 15 years & never recall this sort of problem(fyi- i have never worked on isuzu's before). What could I be missing? Any insight would be very appreciated, PLEASE!!!

( 1999 Isuzu Trooper Stereo Information )

Double check the wiring  click on above the link