Electrical Problem- Rear Defogger not working
 

Ok in the spirit of full disclosure, this is not a question about my mustang. It's actually about my wife's chevy.

The rear defogger is not working. I checked the relay and it is ok. when I check the voltage at the two terminals on the rear window and I get nothing.

To me that would indicate that I have a wiring problem some where in the vehicle before the rear window?

How to I find it or can I?