i have installed an amp and sub in my 04 civic and after i had all the wires run i noticed that i only had about 8 inches of power wire in the trunk to go to my amp. I was wondering if i could add a barrel connector and another chunk of wire so that i had more length or is this going to impeed on the current going to the amp

 

What size wire are you running? Something you can do is install an inline fuse connector and extend from that. Or even a distribution block. This would work well... (at least it is what I use)
Stinger Fuse Block

 

i'm using a 6 gauge power wire and i thought about adding another fuse but i wasn't sure if that was a good idea either since there is one under the hood close to the battery

 

you should be fine doing either a distribution block or another fuse holder. Just Do Not us a barrel connector on that size wire. It will not transfer the current the same a way a distribution block would.

 

If your amp isn't fused onboard, I would use a fuse block. If it is fused onboard, a distribution block would be fine (or a fuse block) Neither will hurt.

 

You should have it fused as close to the battery as possible, adding a distribution block would work, and you can indeed extrend the wiring using a butt connector (or barrel as call it) but to get the current transfer you would need to solder and shrink tube it otherwise you will not get the current transfer. I do not recommend using the butt connector though, instead use the distribution block. As a side note I hope that when you ran the power wire that you ran it through a firewall grommet, otherwise you are setting yourself up for problems from the start.