Look for a plug that goes under the front of the seat cover with 2 wires, unplug that and use an ohm meter to check for continuity within the lower seat heater element (test leads to each wire, one on each). You should see some resistance. If not, it's "opened" which means that the element is NG and needs replacement. Unless you've dealt with hog rings (auto upholstery work), you probably wouldn't enjoy replacing that element itself (seat removed or at least unbolted).
I'd also check that you have 12 volts going to it when the switch is turned on as I've also seen bad switches in S70s.