Brake light/turn signal mod ?

How do the tail light conversion kits for trailer light kits work. I'm making my own tail light housings and want to use one lamp for tail lights, turn signal and brake lights, light the old style tail lamps. This is just a project so I don't want to buy the conversion kit...just need to know how they work so I can make my own.

TIA, Dan

In a word: diodes. If you want, for example, 2 separate sources to operate a common filament, you need to put a diode in line with each of the source lines so that it doesn't short back to the other source. Remember that the old style single bulbs are two filaments in one package, so you do not need to isolate the tail/parking lights, which have their own filament. The brake and signal share a filament, so you need to isolate these to prevent the signal from flashing both sides.

You might consider visiting a junkyard, and see if there's a vehicle there with one of those little adapter boxes installed. It can save time over figuring out the wiring yourself.