You can unplug the connector under the seat, and test if you have continuity (low Ohm value) through the thermostat and the actual heating element (your seat will be cold so the thermostat should be "closed"). The heating element can break/burn out (at least that was the case with the 850 model).

However, since both seats don't work, I suspect it's not the heating elements because it's unlikely both are broken, usually the driver's side goes (first) because it was dimensioned for skinny Euro behinds.

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'97 850 n/a Wagon, 170K mi ... '98 V70R AWD 106K mi