Looking at the wiring diagrams (albeit for yank versions but hopefully should be pretty close), if the mirrors are working, then the main fuse and the relay should be working. The mirrors have their own 10 amp fuse (MIR-HTR), but the power feed for that fuse is via the 40 amp DEFOG fuse and the defogger relay.
If any of the element wires are broken, then most or part of the rear screen heater should still work, so it shouldn't be that unless somehow every one is broken.
So it would tend to suggest wiring to the heated rear screen, or the noise filter just before the heated rear screen, or the connections onto the rear screen. Without taking panels off in my car, I can't see what this filter looks like, or exactly where it is located, but I would assume it will be almost directly by the positive side of the screen heater, or even on the heater itself. I think all you can do is trace the wiring with a multimeter and see where your 12 volts disappears. Checking either side of the screen and its connectors would be the easiest place to start I would think.