adding a third seat to a 1998 xc70 wagon...HELP

I am hoping that one of you can help me..and quickly! I have a 1998
xc 70 wagon, and recently had a surprise baby. We decided to keep the
car and add a third seat. I did the research and bought what is
supposed to be a complete rear seat assembly from E-Bay, seatbelt
hardware included. I dropped it all off with the car at a local
mechanic (not a Volve dealership, but someone who works primarily on
European cars). I just got a call from him stating that he installed
the seat just fine, but is unable to install the seatbelts, and that
he hardware included didn't seem to fit my car.

I thought that all Volvo wagons were set up to be retrofitted with a
third seat?

Are there different types of seatbelt hardware out there, could I have
gotten the wrong type?

If so, how do I know what type of hardware I need, and where can I get
them affordably?

Could he just not know how to get access throught he trim to mount
them, and if so, how would he do that?

Again, help would be greatly appreciated, as the car is sitting at the
shop as I write!

Thanks so much!