Any supply currently in the car is going to be designed to power whatever it is powering, with a tolerance built in for safety. Hanging a high power head unit off of this supply is going to mean too much current draw, so you need to bring a new one in.
I take it you have a high power headunit thats driving 4 speakers?
What I would do is run a supply of decent guage wire straight from the battery (fused of course) to your headunit.
Alternatively, if you are running a distribution block for amps in the boot, you can run a live (again, of good guage size) from your distribution block to your headunit.
