Here is the European to US Trailer converter 3 => 2 w/o = voltage drop caused by diodes.  I have designed this for my own purposes.  = This could be the same schema Mike mentioned.

You need:
2 pcs = change over relays (12 V automotive or general)
2 pcs 1 Amp diodes
2 pcs = electrolytic capasitors
some wire, connectors etc.

And it works as follows:

If the ST wire has + 12 V then both LTS/ST and RTS/ST = has + 12 V.
If LTS has + 12 V then its relay is energized and connects it to LTS/ST.
If both ST and LTS wires have + 12 V then LTS is = dominating.
While LTS flashes + 12 V on/off capasitor keeps relay energized during
off = -periods => same tact and flashing rate as in towing = vehicle.

Selecting capasitor value (voltage rating 25 V preferred):

First measure = impedance of your relay.
If you have automotive relay its impedance might be = 100 to 150 ohms and general relays could be 200 to 300 ohms.

100 ohms = => 3300 mikrofarads
150 ohms => 2200 mikrofarads
300 ohms => = 1000 mikrofarads

These values give about 500 ms off delay to relay.

Schema:
Diode:   (anode)--D|--(cathode)

Capasitor:
 __|__  + terminal
 |____|
  = -------
      |       - terminal

Relay coil:

  = __|__
 |__\__|
      |

Poles of the relay
(drawn as de-energized)

n/o ---o
           = ;  /o--- c
n/c ---o/


GND = GROUND
TL  = TAIL = LIGHTS
LTS = LEFT TURN SIGNAL
ST  = STOP or BRAKE LIGHTS
RTS = RIGHT = TURN SIGNAL

GND = ---+--------------------(white)------------------------ GND
           =    |      TL = -----------(brown)----------------------- TL
           &= nbsp;  |
           &n= bsp;  |   LTS ---+-------------------------------o
     &nb= sp;        |            =     |            =             &= nbsp;           &n= bsp;      /o--- LTS/ST
          &nb= sp;   |            =    +--D|---+-------------+        o/
           &= nbsp;  |            =             __|__        = __|__     |
           &n= bsp;  |            =             |____|      |__\__|     |
           &n= bsp;  |            =              = -------           = |           |
           &n= bsp;  |            =             &= nbsp;   |            =      |           |
           &n= bsp; +---------------------+-------------+      =     |
           &n= bsp;  |     ST ---(red)---------------------------+
     &nb= sp;       +---------------------+-------------+      =     |
           &n= bsp;           &nb= sp;           &nbs= p;  __|__         __|__      |
           &n= bsp;           &nb= sp;           &nbs= p;  --------       |__\__|      |
           &n= bsp;           &nb= sp;           &nbs= p;  |____|            = |           |
           &n= bsp;           &nb= sp;           &nbs= p;       |            =      |           |
           &n= bsp;           &nb= sp;      +--D|---+-------------+        o\
           &= nbsp;           &n= bsp;       |            =             &= nbsp;           &n= bsp;      \o--- RTS/ST
          &nb= sp;       RTS ---+-------(green)--------------o

Hope this will help someone.

Juha Koivisto
88 Vanagon (Caravelle) 1,6TD (old = school bus with 9 seats)
TURKU, FINLAND