1998 P71 rear speakers don't work.

I have a '98 Crown Vic purchased at auction ~4 yrs. ago. It still has the separation barrier, police radio, PA/siren, light bar and wig-wags hooked up and you can still see the outlines where the word 'police' and the 'shield' where on the doors and trunk (the only things I can tell that are missing are the shotgun rack, dash-cam, radar and police laptop). It was used by a small town police department and doesn't have OD or cruise control. It has that factory AM/FM stereo but the rear speakers don't work. I've tried a different factory radio from a 2000 P71 where all 4 speakers are working but still only the 2 door speakers work in the '98. I'm guessing that the rear speakers are disconnected somewhere. The quick-disconnecters at both speakers and at the radio are undamaged and I can't find any cut wires in the dash, behind the rear seat, or in the trunk. Has anyone dealt with this before? Were the rear speakers disconnected/disabled at the factory on some models? How do I find the disconnect? How do I fix this?

Thanks in advance!!!

Moose666