PTO Relay
WARNING: This page is about a different car, the 2006 GMC Sierra and 2006 Chevrolet Silverado. However, it is still accessible from the selected car via links, so may be relevant.
The PTO Relay is located on the side of the underhood fuse block. The secondary side of the relay is supplied with voltage at all times through the Fog Lamp fuse. The PTO switch controls the voltage to the primary side of the PTO relay while the PCM controls the ground.