Thanks billyokco, starting to come together in my head.
Starting to think I might have not left enough length of cable in the engine bay before taping it all up
But I do have to put a fuse there, so I will have to extend it anyway....
Suppose I could always just cut the tape off and pull a little more through.
Kinda cheated with the power cable as it is from an OLD install on a previous car that I found in the loft. I think/hope that it is actually the perfect length. We will see
Anyone heard of "Star Earthing" ... never heard of it myself till the other day, but I ran a cable just in case.
Basically, there is a VERY good earth behind the head unit. Directly to the same panel the main earth is on from the battery(Or VERY close to it) in order to make sure the standard head unit doesn't get any interference.
When you ground your amps in the boot to the chassis, the current has lots of panels and welds to cross to get back to the battery.
So you run an earth cable (very thin, 1/1.5mm) from the new earth you make directly to the main HU earth, that was a small amount of the current goes down it to the GOOD earth, which cuts down the interference/whining you can somethings get.
Obviously what you REALLY want to do it have the amp RIGHT next to the battery/HU and power straight from the battery and earth ONLY to the HU earth.
But in the real world, 99.9% of the time that it just impractical.
Apparently Audiphiles do this "Star Earthing" all the time in Home amps/setups. And can PROVE it makes a difference.
Was very easy for me to run a tiny extra earth cable with the power lead. So I did it, just in case