did you happen to get your wire gauge sizes mixed up while talking? i can see that your ground distrobution block wire is larger then the wire feeding the amp, but you said that the positive amp wire is 8ga, and the one coming off the distro block is 10ga-- which is smaller in diameter then 8ga... while it is typically recommended to run a larger diameter ground back to the battery, at a minimum, run the same gauge wire for both power and ground..
hopefully you straightened out that ground attachment at the battery-- that is putting un-needed stress on the cable to loop it out the top like that.
it is typically recommended to use different color wiring for the power and ground-- esp when running the length of the car, so that they don't get confused later on.
every vehicle is 'supposed' to have a 12v accessory wire. with the advent of canbus, some cars no longer have a specific wire in the radio harness that has this(the radio is turned on by a 'on' command sent over the canbus). there should still be a wire in the fuse box that is switched with the ignition, but it can take some time with a volt meter to locate..
when you zoomed real close to the wire-- is that solid-core wire?
in terms of connection, i am still a advocate of using rca y-adapters off a single 3.5mm jack to feed all 4 speakers-- it is the only method that is guaranteed to work regardless of the motherboard drivers(some don't support the 'enable rear fill' option that is needed for the method in the video to work)..