Crank up the volume to max, measure your current. Multiply it by 12v, than multiply by 0.707. Should be close to the RMS wattage.Stock rear speakers in the sports are 4ohms. I have tested mine with a multimeter.
I experienced the same thing when in went to replace the stock speakers with some Polk components. I can to the conclusion that the stock head unit does not push enough power to run the aftermarket speakers.
If I can figure out how to measure rms with a millimeter I will test it out.
Only disadvantage to using just the rear speaker signals to drive all 4 speakers is you lose your Fade ability.Oh dang. So we have no amp? That's actually good news! lol. Extremely easy to tap what's likely a full range signal on the rear speakers for anything, which means very short input wire runs (besides power/remote).