Most of the time, when people install head units, they just wire the Battery + wire of the unit (usually Yellow) to a small 20 gauge clock wire which happens to be in the vicinity and has 12v on it with the ignition off.
This is common in older cars whose factory radios didnt have a clock or the need for a constant 12v feed and therefore had no constant 12v wire in the factory radio wiring harness (like mine), just a switched 12v from the accessory circuit. And therefore required some poking around to find a constant 12v wire. Most would be smart and at least tap it from the main feed to the fuse box/ignition switch.

But, in most modern head units, the Battery + wire does more than just supply memory and clock power to the HU, it supplies all of its running power to feed all the components when the stereo is on. The accessory wire is to switch the unit on and off, much like a car amplifier's remote wire.
Now if you have just hooked your HU Battery + to that 20 gauge clock wire, you will find that your stereo wont perform that well, and when really cranking it up (and drawing a lot of amps), you might find distortion, dimming of the display on heavy beats at high volume and quite possibly, the "clock wire has just melted" smell.

Some powerful HU's, ones that supply 52 REAL w x 4 or more, or ones with LCD screens, will specify clearly that you need to connect the Battery + wire directly to a heavy gauge wire (#12-#10) and then through a suitably sized fuse and then to the battery positive. Best measure is to put the same rating fuse at the source as there is in the head unit.

Although, its not always like that, and the Battery + can be purely just to supply clock and memory with power while the car is off and the ACC wire can be to supply all running current, although I haven't seen this for a while. This system seems to have come in about 8 years ago, when the colour coding went mostly to Yellow, Red and Black for the power wires.

One way to find out which wire draws the running current, is to run the stereo providing its wired correctly, and first remove the power to the constant 12v wire, if the stereo stop running completely, this is the main feed for the stereo, if it doesnt, it is purely a memory wire and will probably draw no more than half an amp. Likewise for ACC wire, although on both systems the stereo WILL stop when its disconnected. On my Pioneer HU for example, you can clearly see that the Yellow and Black wires (constant 12v and ground) are much larger than the Red ACC wire.

Alternatively, I have devised a solution to the problem, if you cant (or dont want to) work out which one supplies all running power. Its a relay. Look at the diagram I have included, I have put the terminal numbers on the relay terminals to aid in installation.
Bearing in mind, any head unit that draws any current significant enough to need a relay , will probably need wiring to the battery anyway and will probably use the new system anyway.

Hope you found this useful.

Thanks
Ben1990

I wired my head unit with a left-over piece of 5awg. The acc wire is jumped onto the +ve wire, because that was how I wanted it set up. I made sure to connect the acc wire to the +ve wire BELOW THE FUSE IN SEQUENCE. Meaning that the acc wire is fused along with the +ve. I ran the -ve and -ve wires direct to the battery, through the firewall with the exception of 2 fuses on the +ve line. I am not using the speaker outputs, only RCA preouts to the amps. The illumination/dimmer wire is not hooked up, as if the brightness of the LCD on the HU bothers me, I can turn off the screen rather easily. Just make sure to turn off the HU when you get out of the car.

Great tute, if you want to pick apart my wiring job for educational purposes, go right ahead! Welcome to the boards.

I found it useful, cheers!

Although, i'm not sure most people splice their 12v for the permanent power from an existing loom... or maybe they do? I just know it's a hell of a lot easier to run a fresh wire from the battery, through the firewall and down behind the headunit, then it is to go poking around 40 wires to find one that's permanent power and then splice it if the other connection is still needed!