For a stock system you'll need the switch and control relay in addition to a lot of wiring depending on how much of the existing wiring that you have. The stock circuit uses a 16 gauge fusible link so you're probably looking at 12 gauge wiring for the main circuit.
You might check the car to see if the wiring is already there. Pull out the switch for the emergency lights and see if the socket it is there for the rear defrost. A lot of times the wiring is the same in these vehicles, they just didn't add the crucial components.
The purple wire is not used. There should be a Brown/Light Blue wire that is used for the 'on' indicator and is wired to the output of the control relay (the GY/LB wire). The other wires are shown in the schematic.
The wires are probably still connected to the correct spots in the car. Ford probably just installs a dummy switch similar to when you don't have fog lights. You might still have the connector under the dash for the control relay. The connector should be in the vicinity of the fuse box. On an '89 it is just to the right and above the fuse box mounted upside down on a metal bracket. On the 92-93 it looks like the relay is tied up along one of the other harnesses. I don't have any pics for 90-91.
ok update, tried to follow wires through got lost though in the jumble, the purple, black/blue, grey/blue and black orange disappear in the large jumble attach the underside of the dash. took off the drivers side kick panel and i found a male 4 pronged connector with the same color wires except the black orange is a yellow/black. also i see on the new hatch there are 3 grounds, 2 for each side of the defrost and 1 where all the connectors are, this one for the third brake light? and where does it attach to?
The same color wires are used all over the car for various circuits. The only wire I see going to a connector behind the kick panel is the BR/LB wire. The easiest way to verify same color wires is by using a meter.
There is a ground in the hatch for the Hi-mount, hatch popper, and license plate lights.
The wires for the defrost circuit should terminate at a connector under/near the steering column. This connector would be used for the control relay.