You waited 6 years to bump this? That is what I call patience!! I don't have that car but I will try to offer some basic advice. I will assume you have a volt meter. If not GET ONE! A schematic is always nice but some of them can be a little hard to read anyway. The first thing to try is try to find where the wires connect to the rear windshield element. In the old days these were exposed but on mine they are not, so I don't know exactly where they are. Anyway try to find the wiring AT THE WINDOW and measure the voltage with the defroster ON. If the car is running you should measure around 12V. This is because the alternator should be providing around 13.5 but there wil be some drop due to the fairly high current the element draws. If you measure 12 volts or so then the problem lies in the element or the connections to it. If you don't measure 12V then the problem is either with the switch, or some connector or relay in between the switch and the window. Make sure you leave the window connected for this test, don't unplug anything. Sorry I can't offer more help than this! Hopfully someone else can give a more specific answer for you.

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1998 C230 330,000 miles (currently dead of second failed EIS, yours will fail too, turning you into the dealer's personal human cash machine)
1988 F150 144,000 miles (leaks all the colors of the rainbow)
Previous stars: 1981 Brava 210,000 miles, 1978 128 150,000 miles, 1977 B200 Van 175,000 miles, 1972 Vega (great, if rusty, car), 1972 Celica, 1986.5 Supra