my amp comes with a speaker level imput thingy. it looks like a wiring harness. theres a positive and negative for each speaker pluss a ground. what is this thing for? all it says in the manual is it provides connections for a high level stereo source, what does all of this mean and should i use it, whatever it is. thanks

 

It does just what it says, which is to allow your amplifier to accept input signal from your speaker outputs on your head unit, if your head unit lacks RCA outputs. I would use RCA or line level input if you can, and just forget about speaker level input. Line levels have a lot less distortion than speaker levels.

 

Coalyard nailed it.
instead of using RCA pre-outs, if your cd player doesn' have them you can just run the speaker wires to the amp.

 

ok, that makes sense. my head unit only has 2 RCA outputs. right now i'm only amping my back 2 6 by 9's off of the amp because i blew my subs. once i get new ones should i still only have power coming in through the RCA junk or should i use the RCA to run the subs and use the line outputs to run teh 6 by 9's. ohya i have a 4 channel fully adjustable amp.
hopefuly what i just said makes sense

 

so you wanna use your 4 channel amp to run subs and your 6x9s?

 

yeah, that would work right? i have a 380 watt amp. right now i have my front 2 4 inch speakers runing from the head unit, the 2 6 by 9's runing out of the front to channels from the amp and when i find some subs they would run from the rear 2 channels from the amp.

how could i wire it to get the most effective system? i dont know what subs i'm getting yet so i dont know how much power to supply to the rear channels... bummer

 

If you're just looking for volume, then your best bet would be to amp the 6x9's with the subs. However, I don't know if it'll sound very good. The problem with this set-up is that almost all of the power/sound is being directed behind you. This will pull your soundstage back from where it's supposed to be.

I would personally amp the front 4's and use the deck on the 6x9's. They won't provide the same level of volume that the 6x9's would, but the system as a whole will most likely sound more balanced. Generally speaking, the speakers you are using for rear fill are allowed (and encouraged) to be receiving less power than the fronts so that the music sounds like it's coming from in front of you. It's not a problem for subs because bass is non-directional, but your 6x9's reproduce mid and even high frequencies.

Just like anything else though, it's best to try both ways out yourself to see which you prefer. That way your choice will be obvious.