Broken heated rear window SOLUTION!

[morbias]

I would also look at the relay. The light in the switch is on the low current side of the relay so it shouldn't be affected if the heater is drawing too much current. The only way i can see it causing the light to go out is if the internals of the relay are making slight contact with eachother. A test light might not have enough current draw to show it up, but the draw from the fan might cause the circuit to ground out through the relay making the potential difference across the light in the switch zero, causing it to go out.

It makes sense in my head, but that's not a guarantee of anything

[EDIT] when I say current draw, I actually mean impedance, sorry -- man flu is messing with my head. In other words, the fan has higher impedance than the faulty relay and the high side of the circuit will take the easiest path to ground.

 

[malcolmpaul]

Many thanks for the advice fellas, where is the relay situated on a 72 240Z?

 

[malcolmpaul]

After about 6 months finally got round to it! Couldn't understand why it lost power when the heater was connected, it turns out it was dirty contacts in the HRW switch, took it apart, cleaned everything up and, hey presto, now works fine

 

[RIDDLER]

Fantastic news! Mine does the job - not as efficient as a normal HRW, but it certainly helps and you'll appreciate it in the winter months when you steam up!