If we're talking about the dual speaker I showed you, each speaker has a + and a - marked on them.
Take 4 wires and run one from each of those terminals to the floor.
Label them according to what is marked on the speaker terminals.
Call one side 'left' and the other 'right'.
Twist the 2 wires marked +L and +R together.
Twist the 2 wires marked -L and -R together.
Attach the + wire output from your radios' front + output to the + twisted pair.
Attach the - wire output from your radios' front - output to the - twisted pair.
If the speakers are 4 ohm speakers, wired as I said, they are now 2 ohm speakers. They will be louder but will be VERY easily over-driven (distorted).
Fold the wires neatly in a bundle and tie them up behind the radio.
I told you to cut the length to the floor because you may want to change you unit later and I wanted you to have enough to work with when you do so that you don't have to pull the dash pad again.
I presume that the rear speakers are already hooked up? Or, does your unit only have the 1 output?