My rear, driverside, brake light is constantly on when ever I turn on my lights. It works fine when the lights are off but it wont turn off when theyre on. can someone please help?
-The Red Rooster

What shape is your brake switch in? The one behind the brake pedal. Can be an SOB to get to, especially if you have a clutch.
How good is your turn signal blinker? Check the flasher cans on the fuze panel. Might be a short in the column turn signal switch.
If these parts are good, go on to the wiring.
Any fuzes blown out?
Is the taillight housing grounded?

I'd just check the bulb. You didn't post the year of your truck, but I'm not sure if any of them are different. All the pickups/cars I've ever replaced bulbs in (always 76 or newer) required Dual Filament bulbs. If you, a PO or a helper put in a Single filament bulb then it will act exactly as you are describing, it will burn bright when applying the brakes with the lights off, but when your lights are on, the one bulb will be brighter than the other and will not change intensity when the brakes are applied.

Thanks for the replies,
I have a 67GMC swb.
Blink32- You know it probably is the bulb ill check it tonight.
'68 Orange Sunshine- I am going to go ahead and replace all the wiring in about a week anyways. I was just looking to not get pulled over by a cop
-The Red Rooster