Marea Stereo wiring

I've bought new stereo for my "P" reg Marea.
Upon removing the cassette player, it's clear what wires are for speakers - then I'm left with two extra wires (assuming 1 is 12v, 1 is Earth). I just need to know which is which, as I think streeo may be dodgy!
Anyway, one wire is purple & black, other is a greeny colour.

Hi I can't speak for the marea but what you should find is

2 wires per speaker - usually the same colour

1 permanant live
1 switched live (connected tot he ignition)
1 earth

Get a cheap circuit tester from Maplin (it will always come in handy) and you can easily work out which is which

Si