Help - Wiring head unit to battery

I have a massive interior shorting problem in my car, where all the interior lights go dim or the dome flashes and the memory on my factory deck is lost.  Anyways I have temporarily fixed it enough so it doesnt feel like a disco.

Well I now want to install my deck and amp basically a simple system, but was wondering if I could bypass the harness and power the deck straight from the battery or the amp and if so what kind of fuse would I need and what guage wire should I use for the power to the deck, not the amp.

Any help would be appreciated cus I'm dying to get this in but am at a stand still.

Details:

Head unit  - 4x50 watts + cd changer; Amplifier - 4x100 watts; subs; tweeters and midrange.

As long as you fuse the wire within 1-2 of the positive battery terminal, you will be good to go.  I'd check the fuse rating for the head unit and use a fuse that's one step above it.  Probably it'll be a 10 amp ATC fuse so you can just use a 15 amp ATC fuse.  All you really need is an ATC fuseholder like this one: http://www.sounddomain.com/sku/ROCRP7130

And the wire can be 12 gauge if you want, it doesn't have to be 10 gauge.

Of course this is all just for the head unit; you'll probably want an 8-gauge power cable with a 40 amp inline AGU fuse for the amplifier.  Just make sure both of the inline fuses are within a couple feet of the positive battery terminal.

And if you have a GM vehicle you might need to get an adapter post.

Only time when you cannot use the factory +12 volts constant wire is when you're dealing with high current head units like the new Alpine's.  In that case a direct connection to the battery must be done, OR you can do a 4-gauge main wire with the 2-wire inline fuse holder if running an external aftermarket amp as well.  Just make sure that you use big enough gauge wire to account for the added current draw (very little but still) of the head unit., as well as the length of the power wie for the head unit.