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:
http://www1.jaycar.com.au/productView.asp?ID=KG9142&CATID=&keywords=relay&SPECIAL=&form=KEYWORD&ProdCodeOnly=&Keyword1=&Keyword2=&pageNumber=&priceMin=&priceMax=&SUBCATID=

I even drew a wiring diagram:


Hopefully this might help others.

Andrew