That amp takes high level input, so just splice the rear speaker leads into the provided rca adapter. Using that will also act as a remote turn on for the amp.I just saw that your running a stock head unit. I'm not sure if it comes with rca outputs, someone with more experience than me will be able to give you better info.
I don't know much about fuses/fusing but I thought you couldn't have too big of a fuse so I just said 150a because my brx1100 is fused with a 150a fuse. And since the amp is internally fused I think the fuse by the battery is just to protect the wire from catching fire so you should fuse according to your wire gauge and length of run.For an 800W amp? I'd wager you wouldn't be able to open a 50A fuse with that amp.
Yes you are protecting the wire, not the amp.I don't know much about fuses/fusing but I thought you couldn't have too big of a fuse so I just said 150a because my brx1100 is fused with a 150a fuse. And since the amp is internally fused I think the fuse by the battery is just to protect the wire from catching fire so you should fuse according to your wire gauge and length of run.