Mike, being in the medical field I always like to follow the researched guidlines of the national organizations although state law may very slightly. I'll pass along the link to the gold standard on car seat safety. My 7 year old daughter sits in the back seat and uses the regular seat belt in the Hummer but I attatched the clip to lower the shoulder belt.
American Academy of Pediatrics
2003 Black H2, Lux, brush guard lights