Oh right thanks much simpler than I thought only needing one channel. Wont be fitting until January. But I like to be prepared
There are only 2 channels on a car system, R + L (right and left) the (eg) left speakers (front and rear) are fed by the same (left) channel
Strictly speaking,(going "buy the book") you should connect both the L+R channels to the sub, but you can "get away" with only connecting one channel/ rear speaker "at a pinch"
You won't hear the difference with 99% of music, some very early Beatles records may sound strange (ie- where the vocals come out of one speaker and the droms, etc out of the other- an early attempt at stereo recorded on 4-track) and possibly some music with sycadelic? bass- panning effects (pink floyd/ some dance music) but that is rare and normally both channels are fed the same bass