tail light wiring

ok so my friend just bought a really old project car and wanted me to help him wire it up. ive dabbled in wiring a bit. car stereos,alarms and the like but im torn on how to wire up the tails. its a dual fillament bulb and im trying to figure out how to wire up the tail,stop and turn on 2 wires. i dont have a factory wiring diagram to go by so im flying by the seat of my pants right here. theres 4 wires total running to the back. 1 goes to left tail light, 1 goes to right tail light, 1 goes to the lisence plate light and then the last one is tied into all of them. so im thinking the one thats tied into all of them is the ground but how would i make this work? i know im gunna need atleast one relay for each tail light but can someone maybe explain it a bit better for me? or send my a diagram or a website?

nothing? no one can help me with this one at all?