normally this is a easy fix. There is a relay located under each seat. If you don't have power seats I can walk you through it. If you have power seats your on your own. Under the seat you will see two sets of wires. One to the seat belt light, other to the SEAT HEATER relay. It maybe kinda difficult to unhook from the bottom of the seat but can be done. Once unkooked turn key to full on pos., place a.c. temp selector to full heat pos. While holding relay in your hand, turn on heated seat. You should feel a click when you turn on the heated seat. If relay is defective it will click agian within a second. This is the problem with 99.99 of heated seat problems on volvos. Good luck.

Joseph