I had a similar problem with my 92 sc400. All of my speakers quit except the rear and they weren't very loud. This happened about the same time that I needed a AAA jump to start the car. I went home and cleaned off the positive terminal post which was very corroded. All of a sudden I had the front tweeters working, but the main speakers are still out. I checked both of the radio fuses, but they're fine. I suspect that the wiring harness connected to the battery on the plus side may be a clue as to the problem.
Can you help identify the site with your instructions for checling the wires?
Thanks,
Ron