Battery light on and break light on

The tail light assembly contains the circuit boards on each side. Driver side and passenger side each have their own circuit boards when you remove the tail light assembly.

It sounds like a dead short in one of the wires going to the brake lights.

please don't break the brakes. ok?
you double posted this and its already answered.
why do that.?

If the master cylinder is full of fluid, the alternator is not always charging the battery.

You need to check the plug that goes into the alternator to see if it's tight fitting next the brushes inside the alternator have a wear indicator that is a hole in the center it is also used to set them in place with a pin that could be the flickering cause the brushes are solid till they get to the opening so when the alternator is running there's always a break in contact because of the hole so it makes and breaks contact causing the flickering and the charging level is ok but when a lot of accessories are on it can't keep up the charging voltages so the battery gradually goes dead losing power also another spot to look is the power distribution box cable it co u ld be loose and dirty and corroded

The brake light switch is inexpensive and easy to change. It might not even need replacing, just adjusting. Don'y disconnect your battery.The brake switch is ounder the dash where the brake arm can actuate it,

check the gruond in the black wire in the blinker switch if its ok, sometime the after market parts have ashort life time try to replace the flasher switch ( blinker switch)

check your fusible links from the battery

There could be several reasons why your break lights wont work. Blown bulbs, bad wiring or the brake light switch has gone bad.

Sounds like part of a wiring harness is shorting to ground or breaking a ground connection. Check engine to frame ground straps and engine compartment wiring. Look for loose or broken leads esp. alternator and battery wires. Perhaps the power lead to the computer is faulty causing the truck to misfire. Any engine codes?