hi robin, if i was you i would run a fresh cable to the battery for your positive live (the current one will be pretty resistive by now) and then undo the ignition barral cover bit and locate a switched live and run that to the red lead on the headunit. Then for your earthing point, if you take off the gear leave surround bit (the big plastic panel where i think your EQ will be sitting), there will be a bolt there you can undo and use as an earthing point and works VERY well...

bit of a long way but atleast you will know the power supply to the headunit will be solid

bassboy