One of my better posts on Fordmods before it was 'lost' was a 'how to' on adding another Subwoofer amp to an AUII with premium sound.

The reason this is a problem is because the switching line out of the head unit only runs at 5 v and cannot supply enough current to drive even the smallest relay. I found that it was capable of less than 10 mA!

The answer is to use Jaycar's relay kit:
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I even drew a wiring diagram:


Hopefully this might help others.

Andrew