[Rust-O-Matic]

Ok, here's what I would do to narrow it down a little further. First, verify power to each contact in the socket. Remove a light and pull the bulb so you get to the contacts inside the socket with a test light (or voltmeter). Now pull the light switch out to the parking light position (or all the way out for headlights, doesn't matter which for tail lights). Ground the test light and touch the probe end to each of the contacts in the socket. One of them should light the test light up. It sounds like you've got power to this one now, based on your description. On a side note, your front parking lights should come on when the switch is in the parking light position, but go out when the switch is pulled out all the way to the headlight position.

Next, turn the lights back off. Now push the brake pedal down and wedge something against it to hold it there (so the brake lights come on). Go back with your test light and do the same process to both contacts in the socket. The other contact should light up this time, and verify that you have continuity from the fuse box, through the brake switch, to the rear lights. If not, make sure you are getting power to the brake switch under the dash, and follow the wires back toward the fuse box to see where the juice stops.

Once you sort that out, then do the same process using the turn signal. Turn the signal on, and put your test light to the same contact in the socket that the brake pedal made light up. The test light should blink. If not, then check the signal switch and related wiring at the column/dash for problems. If your front signals are working, or at least lighting up when the signal is on (I can't remember if they blink real fast or just stay on solid when a rear one is out) then you can assume the switch and dash wiring is ok, and the break in continuity is probably between the firewall and the tail lights.

Once you verify that each of these 3 items provides power to the two terminals in the socket, you can focus on the ground and the lights themselves. The only thing that can cause the bulbs not to light up at this point is either the wrong bulb is installed (1156 in an 1157 socket, or something like that), or bad ground from the socket to the truck bed (assuming the bed is grounded to the chassis and/or battery). The socket gets its ground from the light housing, which gets its ground from the screws and J-clips that hold it in, like we talked about earlier.

Phewwwww that was long-winded! Good luck. Let us know what you find out this weekend.