I don't have a wiring diagram for your truck but I doubt if any of those wires is a ground. The usual 3-wire set-up is:
1. tail lights (connected to the light switch)
2. left blinker/stop light
3. right blinker/stop light

The ground for all these is the bulb housings which are grounded to the bed of the truck.

These can be easily identified with a 12 volt trouble light you can buy at any parts house.