I had this problem (With an Audi TT, but that's not relevant). You may well find this helpful.

I thought I saw a complete wiring diagram in the FAQ, but I can't find it now. I simply corrected it by swapping the wires over half way+dab of solder+insulating tape.

I can't understand how you did the installation without a harness, your CD changer question is answered here

In before Fabris can get out of bed! I'm off to the pub for a celebratory pint!