See if this excerpt from the service manual helps.

Document ID# 871006
2003 GMC Truck Yukon Denali
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Heated Seats Description and Operation Front

Heated Seat Components
The heated seat system consists of the following components:

Driver and passenger heated seat switches
Driver and passenger seat back heater elements
Driver and passenger seat cushion heater elements
Driver and passenger seat back temperature sensors
Driver seat module
Driver door module
Passenger door module

Heated Seat System Operation
The driver and passenger heated seats have 2 heat zone operating modes with
3 heat level modes and will operate only while the engine is running. The
heat zones are determined by which of the 2 heated seat buttons are pressed,
while the heat level is determined by the number of times they are pressed.
Pressing the seat cushion button 1 time will activate the seat back and
cushion heater elements in the high mode. Each time the seat cushion button
is pressed the seat back and cushion heat level is reduced from high to
medium, low, and off. Pressing the heated seat back button 1 time will
activate only the seat back heater element in the high mode. Each time the
seat back button is pressed the seat back heat level is reduced from high to
medium, low, and off. During heated seat operation the heat zone operating
mode may be changed without affecting the current heat level mode.

The heated seat switches are momentary contact switches and are inputs to
the driver and passenger door modules. Battery voltage is supplied to the
switch assembly from the door module, and when a heated seat switch is
pressed and released a brief switch signal voltage is supplied to the door
module. Whenever the door module receives an active heated seat switch
input, the next heated seat operating mode is determined by whatever the
current mode may be. The seat heater elements are controlled by the driver
seat module using inputs from the door modules through Class 2 serial data
circuit messages. The driver seat module controls the voltage supply and the
ground circuits to the seat heater elements. When a heated seat function is
commanded active, the seat module will switch battery voltage to the heater
element supply circuits, and ground is provided through low side drive
control circuits. During heated seat operation both the seat back and
cushion heater elements are supplied battery voltage. The seat module
grounds the appropriate control circuits for back only or back and cushion
heating modes, and opens or closes the active control circuits as necessary
in order to maintain the desired seat temperature. The seat module relies on
inputs from thermistors located in the driver and passenger seat backs to
control heated seat temperatures. The thermistors are 2 wire sensors
supplied with a 5-volt referenced signal circuit and a low reference circuit
from the seat module. Resistance through the thermistors varies with
temperature causing the heated seat sensor signal circuit voltage levels to
decrease as the seat back temperatures rise. The seat modules allow heated
seat operation only while the engine is running which is determined by a
Class 2 serial data circuit message from the powertrain control module.

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  Document ID# 871006
2003 GMC Truck Yukon Denali