Hi! first of all I think this place is great for sharing information, I live in mexico and just got a nissan quest 1993 and I love all the features it give to me for such little money.

When I got the quest it was still having the factory cassette stereo so I decided to put a dvd player, when I have removed the stereo there where no mark in the cables so there was no way to find out the wiring and I started making some tests,

My advise is pretty simple and easy, you need a tester (is a little bulb inside a pen) so you first of all can test which cables are from the battery and connect those to the new stereo.

and to find out which are the speaker wires all you need is a 9 Volt battery, when you connect both battery terminals to the same speaker's two cables, the speaker pops, so you can hear which speaker are those cables, you must try one cable against all the other but it won't take more than five minutes to figure out all the cables.

by the way you must first check which are the power cables so you don't connect the 9 V battery to those, I don't think you could damage your car but let's better don't take chances.

I hope this information is useful and apologize if my english is not so good, don't forget I'm Mexican.

Viva México!!!!