If you can not find what in 8Ohms 4Ohms will work. I would not try full volume though.
Car manufactures usually have more efficient speakers. What ever you replace your rears with should have a high db at 1Watt at 1Meter. If it does not the factory fronts will be louder than the rears. A total guess would be 92db/WM. My car's PO replaced with aftermarket so I have less volume than a stock system. The other problem is that aftermarket speaker manufactures some times add bass to the speakers. This can be more than a simple bass control can correct. With a stock stereo you should also look for a speaker with a "flat" frequency response from 20Hz-20KHz.
Finding speakers that work well with a stock system is a challenge.
Car manufactures usually have more efficient speakers. What ever you replace your rears with should have a high db at 1Watt at 1Meter. If it does not the factory fronts will be louder than the rears. A total guess would be 92db/WM. My car's PO replaced with aftermarket so I have less volume than a stock system. The other problem is that aftermarket speaker manufactures some times add bass to the speakers. This can be more than a simple bass control can correct. With a stock stereo you should also look for a speaker with a "flat" frequency response from 20Hz-20KHz.
Finding speakers that work well with a stock system is a challenge.