This could be a short to earth or a line to line short. A good way to fault find is to set a multimeter to resistance, connect the black lead to a good earthing point on the chasis and the red lead into the circuit side of the fuse holder. At this point a short to earth will give you a low reading (hence why your current is too high). Then remove the head unit, if the problem is behind the stereo this will more than likely remove the short to earth and give you a much higher reading on the multimeter. If this happens you can then have a good look inside the dash and hopefully the culprit will present itself. If not, PM me. I have a wealth of electrical and electronic fault finding experience and I'm more than happy to help.
This is great adivce.
I have absolutely no clue about electrical fault finding so this will give me something to start with should I encounter electrical gremlins.
Thank you in advance!
