yeah it´s the switch. I´ve seen this switch go on many many escorts... easy to run to the junkyard and pull the part and replace it. It´s a simple like 10 minute repair.
Also, you could have a cable break in the rubber hosing inbetween the rear hatch and the body.... this will also keep things from working on the back hach, like the rear windshield wiper, hatch courtesy light, center mounted brake light, etc.
I don´t think that there even is a relay for the rear defogger...
And this isn´t your problem, I was testing to see if it was maybe the IAC valve, but I don´t think it is... I think it´s an air metering problem or one of your O2 sensors is fuxored up. This will happen on a GT when you unplug the VAF. people say you can´t run a GT without the VAF plugged in, but I´ve personally done it before and it does certainly run, but extremely poorly. The engine shuts out at anything over 2300rpms and is very very unstable.
So this problem goes away when the engine warms up? Does it only happen when you go from reverse into drive? The more information you can give the more we can help.