I doubt Volvo would install the a seat with the thermostatic control and heating elements on a car that wasn't purchased with that option - As I recall heated seats added several hundred $$$. Whereas wiring harnesses are cheap and not worth the trouble on the assembly line to have different wiring. I'll bet you'll probably find the car is wired for driving lights, side turn signal lights, trip computer and electronic climate control - but is missing the switches and electronics to make them work if not purchased with the car.

For example on my 850 wiring it to tow a trailer was easy as wiring connectors where all on the car and I just needed the kit to connect - with no cutting wires, etc.

That said - the heated seat wires under the seat consist of 2 small wires, red & white, going up through the middle of the foam on the underside through a hole about 1/2 inch big. If you can see these wires maybe you got lucky (or maybe you got one that had problems with the seat and the switch was removed to prevent activation I have heard of folks getting an unexpected HOT seat)

Good luck