Testing the seat warmer is easy. With a meter check for continuity. Also before you take the seat apart make sure there is power and a good ground going up into the seat. This can simply be a test light I believe the thremostat opens above 50 F. or so so do this when in car emp is lower then that. The thermostat is a small capsule mounted inline with the heat wire. it is about the size of four or five dimes stacked on top of each other. Most common failure is the wires break going into the thermostat region due to too sharp an angle. You may be able to cut off bad end and crimp on a new end. I find the Saab seat warmers are easier to remove in one piece. They are shaped a little diffenret but they can fit Volvo seats.

Bill D.