Bad ground is the problem with the lights, tail lights are using the dash light bulbs for the ground.
Diagram here:
http://www.therangerstation.com/tech_library/EDiagrams/files/Diagram_turn_stop_hazard.JPG
But not that complicated, Ford used Black wires for Ground at the rear and they should be grounded to the frame rail, so either find those and clean them up or add new ground for the black wires.
I found a '94 diagram, all the tail lights use ONE ground point on the frame, G104, so look for that
I posted diagram here:
http://www.therangerstation.com/forums/showthread.php?t=162630
There also should be a Ground Strap in the engine bay for the Frame, often on/near engine mount, if you can't find one ADD one, just engine to frame.
There should also be a Ground strap on the back of the drivers side head that goes to Firewall, thats the main ground for the cab electrics, make sure it is there, often left off after engine work, and make sure it's connections are clean and tight.
Vehicle frames and body parts are painted first then bolted together, so there is no reliable grounding between these parts, and engine and trans sit on rubber mounts, so no reliable ground there either.
If in doubt ground it out, can't have one too many grounds, but can have one too few, lol.