Maybe I can help here.....I have a 6400 as well and had the same crazy thing with the TV's.....is your surround/audio system turned off? If not, be sure to turn it off. I did that and all I have to do is crank up the regular TV sound and the cabover and rear TV work just fine. I have Sansui TV's in all three locations....hope that helps.
I don't have a brake controller on mine, so sorry I can't help with that one.
I hope you enjoy your 6400 as much as I do...it's a pleasure to drive.
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