That could be a problem.
If your current rear speakers are running off the deck power, then don't add the extra speakers. Decks don't like a 2ohm load - both factory and aftermarket decks. The deck will heat up excessively and possibly thermal protect out (I did that with my old kenwood).

If your rears are already on an external amp you are most likely fine because almost all recent amplifiers are 2 ohm stable.

Short answer:
Don't do it if your speakers are running off of the Deck (Head Unit)
Do it if you have an amp for them.

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