Brake light not working also left side of side lights rear

so left front blinkers fail or not.????? other shoe drops.
fuses blown
wires a mangled LR
lamp socket there a mess.
trailer hitch wires hacked in hear still a mess as all hacks are.
grounds to LR is bad or rusty or mangled wires ground.

Replace that bulb. All those functions are from the same one filament inside those two filament bulbs. One filament is for park lights, and the other is for turns and brake light.

Your rear left side combo lost a ground on the part of brake/signal lamps. Just put one more wire between parts.

Has the vehicle been in an accident on the left rear side. First check the bulb to be sure it is not defective, trade it out with the side that works properly to test. If the bulb works well then My thought are you may have a shorted wire. The turn signal flasher works by heat build up within the flasher the heat is caused by the load of the bulbs, the load heats the metal spring causing it to open the circuit then as quickly as it cools down the circuit closes again lighting the bulbs.
There is also a possibility of a defect in the turn signal mechanism on the steering column. The turn signal switch on many models does three different things on many models that use the same bulb element / or bulb for brake and turn on rear of vehicle. It controls the circuit for brake light by sending power both left and right side rear when turn signal is in neutral position then when turn signal is in left turn the switch cancels brake light power to left side rear and supplies flasher power to left side while maintaining brake light supply to the right side rear, and so on opposite when the right turn is chosen on turn signal switch. The turn signal indicators on the dash normally run off the same circuit as front turn signal bulb only which is also controlled by turn signal switch. Front has turn signals but no brake light.
I think there is an additional load on the left side rear turn circuit and it could be either a wire that is shorted or the turn signal switch if the same bulb element is used for both functions.
Often times if trailer lighting is connected the flasher will also blink a little faster because of the extra load of additional bulbs.

because of no light flasher on left side the front one will go faster .maybe the left rear light is burn. and brake lights are control by the brake switch on brake pedal hardware, might be unadjust

tail light is seperate filament inside the brake light bulb- change bulb

yes, on multiple electrical problems like this the first thing is to check the ground circuits, in you case focus on the left rear

Check your bulbs, and look at the sockets, and connections...sounds like you have corrosion somewhere. Check switches also.

you need a rear left circuit board. they come with new lights, and is generaly inexpensive20-30 dollars

check fuse number 7 in Engine Compartment Fuse Box , if its ok you need change turn signal switch