Brake light on (signal) goes on after starting from a stop sign

any truck that old is super hard to find schematics
to simple voltmeter checks find this cause.
fuses, switch, wiring, and lampsl
fuse = haz/stop
goes to stop switch
then stop sw output pin red/black
goes to DIRECTIONAL switch
if wiring is good and any trailer hitch wiring mess is repaired, then this switch below is bad.
brake wires is GREEN I think. see it?

Power to the brake lights flows from Fuse 12 in the underhood fuse box to the brake light switch. Check for power at the switch on the orange wire. Power leaves the brake light switch on the white wire and heads rear fuse box under the left rear seat. Power goes to fuse 34 Vehicle stop fuse 15A. From there it goes to the left and right stop bulb connectors on the light blue wires. If you don't have power on the light blue wires, you've got a bad fuse or an open somwhere. Two fuses in the stop light cirucit.

Deo, brake light goes out when left signal is on, this is normally a sign of a bad bulb/socket/wire/ground or multi function/signal switch.
You will have to have a good Tech. troubleshoot, diagnose and estimate repair if you can not. Check the attached links,instruction and guides, Good luck
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Might be a leak on your brake booster (this will most probable give you a hard brake pedal or a "sucking sound " when applying brakes) also check the vacumm pipe from the engine to the brake booster. It might have a leak, so when you press the brakes it influences the air mixture.

There is no adjustment. The park lights are on a different circuit than the brake lights.
Do you have a third brake light that works. Its also on a different circuit but works from the same switch. Do your turn signals work. They use the same filament as the brake lights.
Also, the brake light circuit goes through the multi function (turn signal) switch.

Since the brake lights and turn signal lights use the same filament in the bulbs, the brake light circuit will pass through the turn signal switch. When a turn signal is used it cancels out the brake light for that side.

Let me know if your third brake light and turn signals work in the rear.
Also, if youy have not already, check/change the bulbs. The type of sockets those lights are in also get burned so look at them well.

Hi, the likely problem in this case is the turn signal switch. the brake lights use the same bulb filaments as the turn signals,and the brake signal goes to the turn switch. The signal is supposed to pass thru the switch when the signals are off. Replacing the switch requires special tools, including a steering wheel puller and a lock plate spring compressor. If you can borrow these tool, I could walk you thru the procedure.

The brake light in the rear window and the main brake lights are on a different circuit. After the signal leaves the activating switch on the brake pedal arm, it splits. One circuit goes straight to the high mount brake light, the other goes into the turn signal switch. If the rear turn signals work but the brake lights don't, suspect the turn signal switch, not the bulbs and wires. Better to check this before removing and checking bulbs, wires and fuses. It is a common problem with the W-body cars like Lumina, Monte Carlo, Buick Regal, Olds Cutlass and Pontiac Grand Prix. (Approx 94-99 models). Part number is GM 190005031 (w/o radio accessory control) or 19005032 (with radio controls)

THE 3RD BRAKE LIGHT DOES NOT GO THROUGH THE TURN SIGNAL SWITCH.ASSUMING THE 3RD BRAKE LIGHT BULB IS GOOD.U SHOULD CHECK THE BRAKE LIGHT SWITCH ON THE BEAKE PEDAL.SHOULD BE POWER GOING IN & POWER OUT WHEN U ARE IN THE BRAKE

Check ground cable

I had the same problem with my 1996 Mystique about a year ago. The brake and tail lights stopped working, except for the center brake light in the window. Turned out to be oxidation of the terminals at the connector on the turn signal. Yes, that's right, turn signal. The electrical signal to the tail lights goes thru the connector on the turn signal. To fix, I cleaned the contacts and the brake lights started working again. Hope this helps. Cheers, Henry