Ok, that kind of makes sense then. Your two internal speakers are probably supposed to be left/right on channel A, but instead they hooked up right from A and right from B. This would explain the behaviour you're seeing by dumping the sound to the left.
So, up to you what to do about it. Either you can pull the radio and fix the wiring to get left/right inside and lose the ability to adjust inside vs out. Or leave it as is and enjoy your inside vs out, but not have left/right when watching a DVD.
There's no use crying over spilled milk, unless it's on your keyboard.