Stick,
I installed mine on my normal convertible installation per the Mercedes Install recommendation. I think you are saying that the California coupe's bench is lower so the reel will not fit hidden underneath it.
If that is correct then IMHO the only alternative would be to install the reel behind the seat. The only down side to that is the seats would not go as far back. The seat adjustment would be limited. As I recall there have been posts regarding behind the seat install.