Is the de-mister relay reacts (on the terminal board in engine compartment),
when the heater switch is operated?
If yes try to check the resistance (with relay removed) between the relay contact
labelled "87" and ground (chassis), there should be a few Ohms.
The relay has two "87" labelled contacts. They internally connected together in the
relay, but from memory i don't remember which contact is used in the wire harness
@ the terminal board.
If the reading is higher than 5 Ohms, or you experience discontinuity, check the
resistance between relay contact labelled "87" and the de-mister positive feed on
the rear window. If the reading still higher than 5 Ohms / discontinuity then it's a
wiring fault between the relay and the de-mister positive feed.