Hope this info isn't too late
After looking at a service manual for an '04 cav, there doesn't appear to be a separate relay for that system. It looks to be internal to the HVAC control assy - the panel with the buttons on the dash
System description:
Rear Window Defogger Description and Operation
Rear Window Defogger System Components
The rear window defogger system consists of the following components:
• HVAC control assembly
• Rear window defogger grid
Rear Window Defogger Operation
Battery positive voltage is supplied through the RR DEFOG fuse in the IP fuse block to the HVAC control assembly. Ignition voltage is supplied through the HVAC fuse in the IP fuse block to the HVAC control assembly. When the ignition is turned to the ON position, and the rear window defogger switch is depressed, the HVAC control assembly illuminates the rear window defogger indicator and supplies battery positive voltage to the rear window defogger. Ground for the rear window defogger is provided by G302. Ground for the HVAC control assembly is provided by G204.
When the engine is started and the rear window defogger switch is depressed for the first time, the defogger cycle lasts for 10 minutes. Further operation results in 5 minute defogger cycles. The defogger cycle resets to 10 minutes when the ignition is cycled to the OFF position and then back to the ON position.
also, no relay in the wiring diagram either