How to get sound in Ford Mondeo 1996 wiring ?

I have just decided to put back a cassette radio in my Mondeo, as far as I know I've wired it ok, but there's no sound coming from the speakers, I have only got speakers in the front doors, this isn't a Ford original stereo, its an Alpine model (no clue of model number) I don't really want to take the doors to pieces to get to the speakers.