Each switch has three lights; on comes on when the headlights are on, regardless of whether or not the seat heaters are actually switched on. (This is the switch illumination bulb.). . . both of my heated seats don't work although the lights come on, although that may be just from wear.
The others two come on when the seat heater is switched on. they light up when the seat heater switch is in the Low or High heat position.
If the High and Low indicators come on when the switch is in either position, that confirms the fusing and the wiring to the switches. However, these indicators only confirm the switch position and not if the heaters themselves have power or actually working.
Are you sure the seat heaters in both seats don't work in either the High or Low positions?
Are the seat heaters connected? Each seat heater has a four-pin, square connector, probably under the seat. (R44 and R41 in the attached "2004 Seat Heater" wiring diagram.)
It's not unusual for the heater elements in the seat to develop breaks, and on a 17 year old car, it wouldn't be unusual. The continuity of the heater elements can be checked at the connectors.
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