I am looking at getting a used E350 15 passenger that has been converted to a shuttle style with high back seats for 12 and seat integrated seatbelts. The seats are really high. I am wondering about tethers. We have four Radians that really do better with that top tether and I am wondering if anyone knows if Ford will retrofit tethers? I really wanted an NV but we just can't swing the price. I am siked to have found one with a center aisle, but really want it to be as safe as a 15 passenger van can be... TIA