Mate it's just a game of elimination. I guess what Ian thinking is keep it simple. Trace the wires and see where the power is and where it stops.
I had about five different goes at getting the driving lights working on my mrs car. I had fitted them and they had stopped working. Searched and searched. Eventually I realised there was power at the lights but no earth. The earth was tight on the rad support but as soon as I touched the bolt with a spanner they started working. Moral, earth looked perfect but it still needed a tweak to get a contact.