Battery wiring questions...

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I'm getting ready to relocate my battery to the trunk in my bird, but I'm still very new to electrical work and want to make sure I have all of my details sorted out first.

I've heard its a good idea to go ahead and run a ground back up to somewhere in the engine bay along with the positive. How much truth is there to this?

Also, I'm going to go ahead and get a pretty nice battery. What are the advantages to deep cycle batteries?

How much cable should I order? and what gauge?

Also, a thought I had-- I've already run a positive cable from my engine bay to my trunk for my stereo amplifier. Could I just use this cable to reach the engine bay from the battery, and run new ones for the amp to the battery?

Also, I was contemplating setting up a disconnect switch under the hood, just for convenience. This is no drag car, so mounting location's not really important. Good idea, bad idea?