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
              |
              |   LTS ---+-------------------------------o
              |                |
/o--- LTS/ST
              |               +--D|---+-------------+        o/
              |                        __|__        __|__     |
              |                        |____|      |__\__|     |
              |                         -------            |           |
              |                            |                 |           |
             +---------------------+-------------+          |
              |     ST ---(red)---------------------------+
             +---------------------+-------------+          |
                                      __|__         __|__      |
                                      --------       |__\__|      |
                                      |____|            |           |
                                           |                 |           |
                              +--D|---+-------------+        o\
                               |
\o--- RTS/ST
                  RTS ---+-------(green)--------------o

Hope this helps someone.

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