I have done the trailer light setup and the best way as described before is
the solid state "black box" that adapts the 4wire/3-filiments bulb setup
(brake, turn, running lights & ground) to trailer setup. remember most import
cars use the separate turnsignal route while USA cars and trailers use the
same bulb/filament for the brake/turnsignal route, hence the need for the
black box.  remember to take your time and seal the splices well so things
don't corrode in there.

good luck

Kurt Thomas
87 Syncro