[floatingkiwi]

did you check the ground like I suggested? Have you checked your alternator to see if it is OK? Is your battery good?
You've been having problems for a while so it must be something concrete. Unfortunately these things require persistance sometimes.

Try and pull out various ancilliary fuses to see if you can bring your voltage back up to required levels, if so you've found the circuit where the problem is (assuming your alternator is up to scratch). What is the alternator putting out at the terminals?
Troubleshooting - always start with the basics and work up from there.
I would say that your head unit is maybe not detecting enough voltage to turn itself on, and that is why when you used the permanent 12V feed it was OK.

Matt