2004 chevy 1500 brake lights not working

Most of the time the problem is in the turn signal switch. The third brake light circuit bypasses the turn signals.
Someone would need to check brake light power in and out of the turn signal switch.

Since the turns work in the back, that means the brake bulbs are good. Brake lights and turn signal lights share the same filament in the bulb.
There is a fuse for the brake lights. Then, there is a brake light switch on the brake pedal. You need a voltmeter or test light, after checking that fuse is good. Need to check if fuse has power on it. This is one of those fuses that are hot at all times-as long as battery is connected, voltage should be going through the fuse and up to the brake light switch. Check for power both places. The switch on the pedal is a simple on-off switch. When brake is depressed, the spring loaded switch is turned on-passing power on towards the brake lights.
But then the brake wire goes into the turn signal switch before going on to the brake bulbs...however, since your turns work good, I doubt if there is a problem in the turn switch. First check if the brake light switch is sending power on when switch is on.
Let me know what you find. Good luck.

I can almost 100% guaranty the turn signal switch is the problem. Power for the brake lights starts at the brake switch. power then splits from there and the third brake light gets power directly from the brake switch. Since the turn signal and the brake lights are on the same bulb power for the brake lights is routed through the turn signal switch. the switch depending on what position it is in will allow brake power or turn signal power to there respective left or right brake/turn bulbs.

If you hold your foot on the brake pedal and wiggle the turn signal and or play with the hazard switch the brake lights will almost always intermittently start to work. this will confirm a bad turn signal switch which is pretty common on GM vehicle with this style setup.

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feed supply problem somewhere ?but where ? try checking the wiring loom at the rear or as its called in american "pigs tails"

Fast flashing dash signals mean there is a bulb issue-DEFINATELY!

Usually when the third brake light works but the regular lights don't the problem is in the turn signal switch.
I would check for voltage there.

check all related fuses.Fuse panel on left side of dash.

I've worked on these the first drove me crazy. its the turn signal switch every thing runs thru these. head lights, turn signal, brakes. except the third brake light. it runs directly from the brake switch. go figuare

Check under hood fuse box make sure L. trailer light fuse isnt blown.

sounds like your bulbs or your fuse is blown