[Maximum Overdrive]

Its probably the connections if you can wiggle it and it works then it doesnt. To be sure though I would unplug the rear tailight harness, have someone push on the brakes while you use a test light to see if you are getting power to that point. (where the two harness's meet) Just kinda double insurance to rule out the brake switch or bad wiring up to that point. It very well could be your bulb socket itself. After all they do wear with time and lose proper contact as these trucks are pretty old.