Because no one has figured out how to transmit power over the air.
The wires need replaced.
Need to know what wires and which harness you speak of.
You only need the hd wires but then you need speakers! I found it easier to by a HDMI wire off amazon
Soldering the wires together
There are many ground wires in that vehicle, you need to be more specific.
some simple machines do need wires and some dont
yes
It could be in need of a new set of ignition wires (spark plug wires).
Spark plugs and wires
that depends on the car and the brand of wires. you need to be more specific
You need to know the firing order to know where your wires go.
use the existing wires you only need to change the gauge of the wires if amp is 500+ watts if less watts there not much difference in sound quality