If they are full range, which it sounds like they are. Ya, you can just tap into the rear wires. But like I said, you may not be happy with the results. The original speaker is 4x6, and I'm guessing the boxes you have probably have a 8,10 or 12 inch speaker. The factory head unit won't have enough power to drive them boxes as well as even a little amplifier would.
If it was my truck, I'd find a little amplifier and wire that in as well. Maybe something that's a legit 100w amp will make all the difference in the world. And with all these new digital amps on the market, they are cheap and tiny.