1997 4.3 4x4 Chevy Blazer LS:

The rear defrost works, but it seems that only a few of the heating elements fire up. Any ideas as to why this might occur?

 

Those lines are conductive, and it works by passing a current through them to heat up the back glass. Might have a few lines get scratched, and "broken." Not a complete circuit, so no current.

I think they sell conductive paint to repair them, if that turns out to be the case

 

Awesome. That sounds like an easy fix. Do you happen to know if I can get a multi-tester on the conductors, or will I have to scratch some material away?