okay, i have a trailer with some electrical problems. it seems easy, but it's not... been trying to sort this out for weeks.

I've wired everything correctly and still, parts of the lights are working on one truck and different parts working on the other truck.

truck 1 96 f150:
brakes, tail lights, right turn signal, surrounding lights work
left turn signal doesnt work

truck 2 2003 jeep grand cherokee:
brakes, tail lights, right turn signal, left turn signal work
surrouding lights dont work.

what the hell is the problem? i've exhausted my brain trying to figure this out.