So your Ranger didn't come with rear speakers installed?
Ford prewires a lot of accessories but never installs the components (such as cruise control and fog lights).
I bet your dash harness was prewired (seeing as it has the correct wires in the radio plug) but your roof interior harness wasn't. For 2000 Ford made 4 different harness for the roof interior; super cab w/premium sound, super cab w/o premium sound, reg cab w/ premium sound, reg cab w/o premium sound(this sounds like the one you have).
If you take out your drivers seat and peel back the carpeting past where the seat was you will see where the dash harness meets the roof interior harness (if I remember correctly there is a plug that goes through the floor board and one that connects with a flat rectangular connect, the rectangular connector is where the dash meets the roof harness).
What you need to do is check at the meeting point for the wires to be ran all the way from the radio to this plug, and also to see if the wires are going from the plug toward the back.
When I get home and get access to my wiring diagrams for a 2000 I'll give you some better instructions (later tonight).
My bet goes toward your dash being right but the interior roof being not wired for it. In that case you either get another roof harness (has to be for a 2000 reg cab model, no other years will be a direct fit), or repin the connector and run the wires yourself (cheapest way).
I may be able to get the connectors and wires for you depending on your trucks situation.
Quality Engineer (Electronics) for Kia Motors
University of Tennessee Electrical Engineering Graduate
2000 XLT 2.5L A/T