It's true that the speakers will have the +/- somewhere on them, but it wont matter which way round you supply the amp unless you have some serious bass coming out your main speakers, as the sub will be 180 degrees out of phase and will cancel out the bass.
If your using the crappy renault speakers, it wont affect the bass. In fact if you do put it out of phase, it wont be boomy, where equal frequencies are adding together and creating peaks.