If you do it, you will assume liability for anyone in the back seat.
They may be factory made parts, but they were never tested/approved in that configuration by the DOT.
The car's occupancy rating is 2. If you get into an accident with more than that number in the it, good luck.
2004 Honda Civic Hybrid "Eeyore" @ 49 mpg
-MaxIMA, OBDIIC&C, IMAC&C, Lower grill block
RIP 2000 Insight, 40k miles @ 69.2 mpg
2000 Insight, AKA "The Basket Case" or "The Go Pod" @185k miles @ 63.5 mpg
-IMAC&C, OBDIIC&C, Scott's aero panel, grid charger, cheapo sound system