The solution is using a "bi-stable" relais. This is a relay that contains two solenoids: one to close the relais contacts and one to open them. If both solenoids are de-energized, the contacts stay in whichever state they're in. In connection with the circuit shown here the current drain on the battery is very low.
When the defogger switch is in the center position, the voltage at the switch will be approximately 6 volts. The first two transistors (T1/T3) will be conducting, and the two relay drivers (T4/T5) will be "off", so the circuit remains in its current state.
If the defogger switch is set to the "off" position, T3 will switch off, thus turning on T5 and energizing the "on" solenoid of the bi-stable relais.
If the defogger switch is set to the "on" position, T1 stops conducting and switches on T2/T4, which energizes the "off" solenoid of the relais.
Getting a defogger switch is not difficult, however the I/P faceplate is not available anymore. The faceplate you will receive from your dealer will not have any openings for any switch, so I recommend getting the faceplate along with the proper connector from a junkyard.
Hint: if you hook up the I/P illumination as well, the switch will look stock at night as well...
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