Ok.... Leave 6 volt bulbs in TD....
On 12 volt tow vehicle get a 7 pin socket put on it and have it wired by the accepted industry standard wiring including the 12 volt supply from tow vehicle battery. .... Take a circuit box and mount a female six pin female connector into box (output for the box to trailer). Bring a tail from 7 pin tow vehicle connector into box....
Get a 12 volt relay for each light function ( tail- stop- turn -???)... wire each relay (relay coil - "operate path") from 7 pin plug. The 12 volt from lamp circuit (tow vehicle) will operate relay when lamp is functioning, Connect other side or relay coil to ground... On normally open contacts of relay wire the output ( 6 volts) of solid-state 12 volt to 6 volt voltage drop circuit (12 volt supply from tow vehicle). Wire other relay contact to appropriate lamp on trailer... All circuits tow vechicle/trailer use common ground.
I state using 7 pin plug so it can move from 12 volt vehicle to 12 volt vehicle, and 6 pin plug on trailer will only plug into "Box" to serve 6 volt lamps on trailer or from corresponding six pin plug on Model "A"...
I state use 6 pin plug on trailer as a reminder that trailer and Model "A" are wired for 6 volt operation.
You can also carry 12 volt through box so if trailer has 12 volt internal lighting you can charge the 12 volt battery while towing with 12 volt tow vehicle... If you are towing with Model "A" you will have to charge 12 volt trailer internal system with shore power.
Concept is really simple, only real technology is 12 volt to 6 volt
solid-state voltage drop circuitry.
http://www.powerstream.com/dc-1206-11.htm
Simple explanation is 12 volt vehicle "lamp function" also operates relay. When relay operates it completes circuit for 6 volt lamp function on trailer...
Dale