Adjustable seat heater operation -

The seat has a heating coil and a temperature sensor (thermistor). The heating element and temp sensor are wired to the seat controller (the four position switch in the dash).  The seat controller contains some electronics and a relay.  The controller has a four-position switch - Off, and three temperature settings. 

The controller compares the seat temperature (from the sensor in the seat) with the setting selected.  If the seat is cooler than the selected temperature, a relay in the controller closes, and supplies 12 volts to the seat heater element.  Once the seat gets a few degrees above the selected temperature, the relay opens and disconnects power from the heater element. 

The element is a zig-zag loop of stranded wire that has a low resistance (the entire loop only has a couple/few  ohms of resistance).

The thermistor has a negative temperature coefficient.  That is, as the temperature goes down, the resistance goes up, and vice versa.  You can check it by putting it into a cup of ice and monitoring the resistance between the black and blue wires at the connector plug; the resistance should increase from 1000 ohms or so to 2000 ohms or more as it cools.

All of the electronics are contained in the seat heater controller. The controllers are swappable left and right (all the same part number), and Ari is pretty sure they're the same between the 9000 and those late model OG900s with adjustable heated seats.

The adjustable heated seats do not have any switches to detect seat occupation.  You won't feel heat immediately when you sit in a seat that's been powered, and you won't feel much if any heat if you just lay your hand on a seat that's been powered - it takes some contact pressure to transfer the heat through the seat cover (true for my leather seats, anyway).

The older heated seats were strictly OFF/ON.  The passenger seat had a pressure switch to detect occupation; if the seat is unoccupied, the open switch  keeps the seat heater off.  I believe that the older seats also had thermistors that cycled the heaters on and off.