For 740's and 940's I believe you'll find there's a relay under the seat that cycles the heater elements on/off until an internal thermostat kicks out. One for each seat. They sometimes seem a little cranky, but may actually work fine at low enough temperatures. The passenger seat also needs to be occupied as there's a weight sensing switch under it. You should be able to detect heat under a cold derriere in about 2-5 minutes. Warming up the passenger compartment before you get in (especially using the floor vents) will often fool the thermostat. The light in the console switch means that power is getting to the switch, but not necessarily all the way to the relay. Normally the wiring is okay unless you've been opening connectors.