The following instructions and the four small photos come from an article by David Roe [see his web page].   Thanks David.   I contributed the animation above and the the large annotated photo at the bottom of this page.

1. You need to scavage a few parts from a late model Ford tailight socket.   I suggest you pick up a used socket at a junkyard.   New ones can cost as much as $35 (Ford P/N F7ZZ-13411-AB).   Take the socket apart by prying the cylindrical collar from the rear cap (it's just a force fit.)   Then remove two wires with pins at one end, as shown above.   These wires will be used to modify the inner sidelamp sockets in the left and right tailight modules.

2. Remove the trunk liner to get at the four nuts holding the tailinght assemblies on the car.   When those nuts are removed you can unplug the wiring harness, push the rubber pass-through grommet out toward the rear, and remove the tailight assemblies from the car.
   

3. Remove the sidelamp socket (shown above) from each assembly.   Notice that it has two stock wires with pins already in place.   You are going to insert one of the wires you scrounged in step 1 (also shown) to make a connection to the second filament in this socket.
   

4. Take the new wire and push its pin into the hole provided so that it makes contact with the unused electrical tab.  
   

5. Use a razor blade to make a slot in the rubber strain relief plug for the new wire.   Then push the stain relief plug into place.

6. Reinstall the cylindrical collar, the white rubber gasket, and the original 2-filament bulb.   Then reinstall the sidelamp socket into the tailight assembly.

7. Splice/solder the new wire to one of the existing green/red brake leads coming out of the other brake socket at a terminal marked "MAJOR", as shown below.