I plan on adding some things to my car and I don't want to put to much on one wire so I was wondering, how do you put in another wire for another 12V source?

A good place to connect to is the horn relay.

What are you adding? Amp kits usually connect to the battery - my Optima has both lugs and screw-in terminals so I can (and do) have four battery cables hooked onto it. It's not pretty but it works. Some day I'll straighten the mess out...

I plan on adding some things to my car and I don't want to put to much on one wire so I was wondering, how do you put in another wire for another 12V source?
What are you adding? Do you need a extra source thats hot all the time or only when the key is on? You could just add a auxiliary fuse box, painless makes them http://www.painlessperformance.com/webcatalog/cat-fuseblock.htm They come in 3 circuit and up. For higher current items (ie electric fans ect...) they should be on their own circuit with a relay. The best place for a source is the horn relay buss bar.

They also make a terminal screw that fits a side post battery. Sort of looks like a 3/4 bolt head that screws through the battery cable to hold it on. On the top of the bolt head is a smaller bolt that comes out. A lug goes under the smaller bolt. Available at the auto parts store for maybe $4.00. Fuse the wire on the lug near the battery.

I plan on adding power windows in the future, I figured I'd set up the wire and cap it off untill I'm ready for it, a security system and maybe power locks if I can figure some stuff out. It is my daily driver so these things are for the convienence. There is not a spot on the fuse box where you can just plug a wire into it somehow?

power windows tend to be on switched circuits while alarms and locks will use constant sources ... I would add a 2nd, maybe 4 circuit fuse box off of a constant 12 volt sources ( not a fused source though ) It won't hurt if windows are on constand sources, but if you want them switched, use a 30 amp relay off primary fuse box for switching and power the windows still off of the 2nd fuse box ... use appropriate size wire for fuse box and power windows ...alarm and door lock can get by a size smaller

Ok thanks, now I have a better idea of how to go about this stuff.