amp install questions

1) I would recommend a 4 gauge wiring kit. You can use the American wire gauge chart to determine the correct cable size for you application https://www.the12volt.com/info/recwirsz.asp

2) I would use 12 gauge speaker wire but 10 won’t hurt

3) Yes any fused switched + 12 volt will work

4) You will get a cleaner signal if you use the low inputs (rca) vs. high inputs.

5) There should be 2 channels on your high to low converter so you will have to tap both rear speakers.

10 gauge speaker wire is a lot bigger than you need.  12, 14, or 16 will work fine for those subs.

Which model # amp is it?  Cuz the way to tell what size power cable you need is to look at the amp's specs for current draw as well as its fuse rating.

I always recommend using an external line output converter because they're compatible with more vehicles' stereo systems than the speaker-level inputs available on most amps.  When an amp has speaker-level inputs that basically means it has a line output converter built in to it, but these unfortunately are almost always not as good as using a dedicated external line output converter.

You're going to have to get a switched +12v signal from the head unit for the sub amp so you'll already be working on wires in the radio's harness.  It would be easier to just grab your signal there by tapping in to the speaker wires corresponding to this diagram.  Any of the ones with plugs that look like yours will work cuz they all use the same speaker pinouts even though they may be different vehicles.

If you tried to tap into the rear speaker wires you'd have to take the whole rear side panel off to get to the speaker, and then you'd have to remove the speaker to get to its wires.  Just a lot easier to make you hookups at the back of the radio.  Make sure that you run the RCA cable from the line output converter to the amplifier, down the side of the vehicle opposite the power wire (i.e. keep them as far apart as possible).

You should be fine using the 8 gauge wire, As far as the subs go are they DVC or SVC do you have model # for the subs?

Each sub is SVC(single Voice coil) 4ohm. You will want to wire your subs in parallel http://www.jlaudio.com/tutorials/wiring/index.html#2svcp for a 2 ohm load on the amp. The amp will put out 400 watts total 200 to each sub.