I use Sound Solutions in Chantilly, so they already have some experience with the G4 (including a rear-view camera, but I don't use ODBII or steering wheel controls).
14508-B Lee Road
Chantilly, VA 20151
(From Route 28, take the Winchester exit for Route 50, and immediately(!) get in the left lane for the first intersection, then take the first right and turn right into the parking lot.)
(703) 631-4104
http://www.snd-solutions.com (not much there)
John