Thanks to JHC for the idea on this one.
I saw this done on Hangman's truck at his house, and liked it.
The one thing that bothered me is the extra current load put on the trailer lights to run the turns/brake (9A extra current).
I tow a boat, run sail panel leds, and a line of fire off of the turn signals on the trailer harness already, so pulling more current was not a good idea. Also, cutting the existing wires and running from the trailer harness was not an option, if I blow the trailer fuse in salt water, I don't want to loose my brakes/turns.
So, I modified the diagram to use relays to keep using the brake/turn signal power, but trigger the crossover between the two with the relay triggered by the trailer wires. The relay only draws 30mA off the trailer wires to run the coil, so no big current drain.
This circuit works exactly as the JHC diode version. When you hit brakes only, all 4 tail lights go on. When you hit directional only the 2 lights on the directional side will flash. When you hit the brakes and the directional, 3 lights will be on solid and the lower one on the directional side will flash.
I used 2 radio shack 275-248 12vdc coil,10A @ 24VDC relay, and 2 packs (4 total) 276-1661 diodes.
The diagram shows the pin numbers to use on a standard 12V automotive relay.