I know this is a seriously old thread, and that it's not proper to respond, but I felt the need.
I purchased a Panasonic RE-8840 last week, with a quad eight-track. Well, I simply couldn't get the rear channels to work... I sprayed everything with contact cleaner: The volume knob, selector switch, balance control, etc. I also went inside the unit and spreaded the internal switches. However, after a full day I was stumped.
When I gave up and turned the unit off, I noticed the sound would get louder in the incredibly weak and staticy rear speakers. I didn't think it would be so easy, but I sprayed some deoxit in the power switch. It worked.
Within moments and a few presses, ALL the speakers came to life. Problem fixed.
This is a decent and warm sounding little compact system. I have ordered a replacement 8-track belt (the old one had turned to dust and fragments) and this thing will be my office music system by the end of the week.
The only thing left to do is replace a fuse light and the three bulbs that illuminate the am/fm dials, and the balance indicator.