You could test it for continuity and try and find the fault, but that may be abit advanced, so the best thing fro you to do would be run a new length to the speaker and join it behind the stereo, just follow the old cable route or you may be able to cut corners and tape/twist the ends together and pull the old cable through.

I've got no idea where the cable goes hahaha, wouldn't know how to follow it behind the head unit either??

You did fix the split wire before you swapped them around didn't you?

And the rubber tube is the thing you see when you open the door.

http://www.tigerstyle.co.uk/phpBB3/viewtopic.php?f=10&t=58802

That shows you how to run new speaker wire through but all you need to do is pull back that rubber tube see if any wires are split if they are put new wire in. If you lived closer I could have it fixed with in 30 minutes.

What's the split wire?