Installing Seatbelts In 2004 Toyota Sportrider

Technically, this vehicle seats 7. It came with 4 seats that have belts-- driver, passenger, and two in the middle row (one by each door, the seat in the center of the middle row does not have a belt). There is also seating for 2 in the back (need to climb in behind the middle row). The back seat can be folded up and hooked onto the headrest of the middle row to free up room in the back. And when it is down, it there is a hook in the floor which it connects to. We would like to have seatbelts installed for the two seats in the back. After stopping by the Lopburi Toyota dealership, they said it was not possible for them to do it and that we would need to take it to one of the car accessory shops in town.

I remember seeing an ad for a used Sportrider that said it came with seatbelts for all 7 seats, so I'm fairly confident that it is possible.

I would like to hear from anyone about their experiences in having seatbelts installed at the car accessory shops. Do these shops know what they are doing? (i.e. are they going to do it properly so the belts will serve their purspose should the need arise?)

And are there any other Sportrider owners out there with seatbelts in the back row? If yes, were they there when you bought the car (and where did you get it)? Or did you have them installed later (and where did you have this done)?