NO. The factory amp filters the speakers at different frequncies. The only frequency suitable for a sub would be those sent to your 2 rear deck speakers. If you replaced those with a sub, you won't have enough power (less than 50 watts rms). In addition, you might have some impedence inefficiencies.
Due to the stock amp's crossovers for each speaker, you really can't replace the speakers either because if you buy coaxial, you will not have the right range of signals. No matter what you do, you have to add a small amp at least for any upgraded sub, coaxial, or component speaker sets. Its never simple..........sorry.
If you could match the impedance with replacement 6x9 woofers then that would probably be better than the stock speakers. I believe the stock rear woofers are 2 ohms each. So if you replaced with 4 ohm speakers then you would cut the existing power to the new speakers in half.............and if they are good speakers then they would probably require more power than what is going to the stock speakers. I am not saying that it won't work, but if you don't match the impedance you will get minimal gains with better speakers. 2 ohm 6x9's are not too common.
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