You are correct, the wires for the rear defrost/heated side mirrors are part of the connector for the Integrated Control Panel (ICP). They are part of connector C237. I've posted the schematic for the rear window defroster at
http://www.members.aol.com/Rod241473...ff/defrost.pdf and connector face/pinout diagram at
http://www.members.aol.com/Rod241473...Stuff/C237.pdf
The GEM module is located between the firewall and the Instrument Panel fuse panel. The fuse panel rides piggyback on the GEM.
Per the schematic, it appear the rear window defrost/heated mirror relay is located in the instrument panel fuse panel.
-Rod