I have been trying to do a little research on a similar question. From what I am reading, to pass inspection, the car needs DOT approved harnesses. Most after market harnesses are SFI approved but not DOT approved. Not sure if this would apply to the back seats. Does anyone have any comments about that? Is this true?
I do know that if they are not installed correctly, they may actually do more damage than good.
I also know that Schroth make some DOT approved harnesses and you may find a solution from them.