I do not know if this is reduntant if the information is some where else. Her goes:
Trailer lights I found at the following Website:
http://www.easternmarine.com/em_store/lighting/
L.E.D. bulbs require min. and max. voltage with a certain currect level to maintain a brightness level set. Usually most people use resistors (cheap ) to limit the current flow to the L.E.D. ~20mAmps. You can use a circuit or device call a CONSTAINT CURRENT DIODE to replace the resistor with so that the current is at the level desired so here is some some tecky infomation below.
http://unclean.org/howto/led_circuit.html
:-) Pat