This is the list of all IDs which determine what a light does on a vehicle in GTA V, these IDs must be set in vertex mode in Zmodeler3. Select your light, switch to vertex mode, select the part of the light that should light up, go to Properties -> General -> External State -> ID and change the 0 to one of the following IDs:
Running Lights (Always on) = 0 (Daytime Running Lights)
Left headlight [headlight_l] = 1
Right headlight [headlight_r] = 2
Left taillight [taillight_l] = 3
Right taillight [taillight_r] = 4
Front left indicator [indicator_lf] = 5