Are you using an amp??
Id say forgetting the amp question you have some bad connections somewhere, check they havent wired from speaker to speaker too as each speaker (For example on your outgoing from the headunit will supply 4x speakers) 2x rear, 2x front
However it is possible to run in paralell if your using 4x rear speakers etc.
So to simplify, pop out your head unit, check that there isnt one cable (Of two cores) coming from example the two purple wires but supplying both speakers.
Id say its more likely to be a bad connection somewhere.
If you have loads of joins, buy a decent length of cable from Halfords (Think its Auto leads stuff, the blue one) re-run to all speakers and bullet connector from loom to cable
Il try get some pics up mate
Hope this helps