Cheaper alternative to metra upgrade cable
 

I did what was stated here:
http://www.mazdas247.com/forum/showthread.php?p=4688300

You can get the cables from:
Digikey
Mouser


Copied over the instructions to modify the cable are:
Just make sure you orient the cable so the black wire goes into the ground of the board. You should be able to ohm this out by looking at the negative polarity side of the large tantalum capacitor (the side without the stripe). You could also ohm it out to the vehicle ground connection I believe. This should allow you to properly orient the connector. Then you can just follow the instructions in this thread:
https://www.rx8club.com/series-i-interior-audio-electronics-24/metra-kit-update-need-cable-look-here-172024/