get ready to connect my amp to the car radio kenwood, authentic and found an entry-level speakers in my amp (Kenwood KAC-749S), but I have yet to find a way to unite the power lead?

Hello

First question do you have a Radio 2 Power Supply?
Red Fused and switched,
Yellow No Fuse and permanent.
With only 1 Earth?
If so, and most are now (secure), then the better you will do is to insert a pin 4 relays
The pins of the relay are normally marked 30 85 86 87.
Red power wire between the fuse and radio connects to 30
Earth connects to 87
New power wire (fuse) connects to 86
Amp wire to connect to 87.
Amp Earth connects to a good earth
If you mount the relay in the dash do not forget to let the In-Line Fuse where you can get it.
This means that your amp will turn on / off with your buddy.
Before the 2nd power has become aware that you may have inserted the 30-pin relay on the ground wire and left the amp on and it would turn On / Off with the radio.

this product costs 300 dollars for the two 12 inch subwoofers, with the box, and a 1000 peak output @ 4 ohms amplifier.
Is this a good system?

make sure the subs are either 8 ohms or have dual 4 ohm voice coils otherwise you'll never get the impedences to match.

Other than that I can't tell you much. Othe helpfull info would be ported vs. sealed box? RMS power output of amp (much more important than peak power) RMS power handling of subs, and type of sound you want.

A 1000 peak amp will RMS at 500 Watts at most. That's 250W per 12" sub, which is not a lot. With a sealed box, this will make a very muddy muffled sound that will only be loud near the boxes resonant frequency. With a ported box, you will get something more usable, as there will be a resonance for the box, and one for the port, giving a broader frequency response.

Honestly, if the power is 500W RMS as I suspect, I'd run 10's instead of 12's if you want it to have any sort of respectable hit to it.