Audio Systems: No sound from speakers - Explorer, pioneer unit, premium systems

Question
I'm installing a Pioneer CD unit into a 95 Ford Explorer, replacing the factory installed casette unit.  I've used the proper (I think!) wiring harnesses.  Unit gets power, but no sound from any speakers.  Fuses for the Pioneer unit and the Explorer (two fuses associated with the player/sound system) are okay.  I believe Ford refers to the old unit as their "Premium Stereo Casette".  I don't think there is any factory installed amp.  

Answer
Doug,
Please give me the complete model number/s off the factory stereo. With the model number I can tell you exactly what type of outputs the stereo uses. Most premium systems will use an external amp.

TOM,
T&D ELECTRONICS
tdeser@zoominternet.net