GM Service Manual Online
For 1990-2009 cars only
Figure 1:

Instrument Panel Switches, Center Forward View


Object Number: 1400889  Size: LF
(1)Outside Heated Mirror Switch (DNN)
(2)High Idle Switch (UF3) or Power Take-Off (PTO) Switch
(3)Exhaust Brake Switch (K40)
(4)Rear Axle Driver Control Switch - Two Speed (HPZ and Automatic Transmission)
(5)Traction Control Switch (NW9)
(6)Overdrive (OD) Defeat Switch (Allison® LCT)
(7)Marker Lamp Switch (JE4, RQ2 and U86, or JE4 and RQ3)
Figure 2:

Forward of the Center Console


Object Number: 397154  Size: LF
(1)Rear Axle Driver Control Switch - Two Speed (HPZ and Manual Transmission)
Figure 3:

Front of the Crossmember, Forward of the Rear Axle


Object Number: 399764  Size: LF
(1)Rear Axle Shift Relay - Two Speed (HPZ)
(2)C404 Rear Chassis Harness to Axle Harness Connector
Figure 4:

Front of the Left Rear Axle Housing


Object Number: 399756  Size: LF
(1)Rear Axle Shift Motor - Two Speed (HPZ)