I remember when I re-installed the rear seat in my '11 RTS after adding a Bazooka Sub I stripped one the bolts. There are instructions on the order to install the bolts so as to prevent that. At the time I just took the bolt to the store and got a replacement. This thread may help with the size.
http://www.ridgelineownersclub.com/forums/5-interior/26779-seat-belt-bolt-threat-size.html
http://www.ridgelineownersclub.com/forums/5-interior/26779-seat-belt-bolt-threat-size.html