Travis.
Here is part of the California seatbelt law...
(f) Every owner of a motor vehicle, including every owner or
operator of a taxicab, as defined in Section 27908, or a limousine
for hire, operated on a highway shall maintain safety belts in good
working order for the use of occupants of the vehicle. The safety
belts shall conform to motor vehicle safety standards established by
the United States Department of Transportation. This subdivision
does not, however, require installation or maintenance of safety
belts where not required by the laws of the United States applicable
to the vehicle at the time of its initial sale.
The way I read this, you aren't required to have seatbelts if it wasn't the law when the vehicle was first sold.
