I did this mod recently.
However, I would advise continued caution when using the rear de-mister. You will be putting a relay very close to the fuel vapour recovery system, introducing an explosion risk that wasn't there before (albeit a very, very, small risk). Also, if the relay fails to auto-cancel the new fuse in the boot could ignite (after some time)...right next to the fuel tank. So /always/ turn off the demister as soon as the screen has cleared.
I may be off the mark, but erring on the side of caution is never a bad thing when it comes to fuel and DIY electronics!