I have a forward facing seat in the back of my E90 in the center position. I used both the latches (sides and top) and the seatbelt. I don't see where it can hurt to have more support so why not use both?
I've read the fact that NHTSA tests with both the seatbelt and the latch system but not both together and for that reason, you shouldn't use both since it hasn't been tested in that setup. Seems like BS to me. If the seat is strong enough to hold up to either, a second strap can't possibly hurt anything.
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