where to start ( rear defrost)

i have an 88 with rear defrost that doesn't work, it seems that i will be on 3rd shift come winter time and i think it will be nice to have a working rear defrost. the switch itself seems to work all the lines seem to be in one piece all the way across the window and the little plugs are still soldered in place. i'm thinking there might be a relay that could be bad, if so where is it?

basically i'm looking for a good starting point to see if i can repair this.

TIA