HU power line to Battery

I just got my new Clarion DXZ845MC and it has been years since I've done an install. I'm not used to dealing with so many power lines coming off the HU. The one that I have a question on is the power line they require to have connected to the battery. I have a distribution block with a 4 gauge feed coming from the battery. It is located under the passenger seat of my Civic Coupe. Out of it comes the 8 gauge line for my amp, also located under the passenger seat. I want to use one of the open feeds from the block for that HU power line. The power line is only like an 16 or 14 gauge, would it be OK to splice an 8 guage to it and feed it off my distribution block? Any advice would be appreciated.  

Also, I'm installing kick panels with 5 1/4" Fosgate Fanatics in them.  My problem is I can't figure out where to put the darn crossovers. I am thinking of just putting them under the driver's eat. Anyone have any ideas? There just isn't any room in the Civic to put them close to the panels.

Thanks.

I'm at work right now but I think there already is a red power wire, a yellow power wire, and then the "battery" power wire. One of the other ones are considered the constant wire I believe. That is what I meant by so many dang power wires.

I was thinking about trying to put them in the kick panels I just didn't know if that would jack the sound up in anyway. Thanks for the help.