The easiest way is not the best way. The easiest is to obviously tap into the rear speakers. This in turn doubles the load, and your now pushing 2 ohms with your stock HU. I don't think it will like high volumes for very long periods of time.
The best would be to power them off an amp. If you do so -- you will want to also power/upgrade your front speakers. The best sound stage should still come from the front. The best would be to also purchase a new rear deck cover -- with the grill covers built in.
If you search on here, several have tried adding these speakers. In the end, it wasn't worth it. If your planning on going "all-out", it may be worth it to add 6x9s in the rear -- and simply do not use the rear door speakers.
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