Brake light is off

Did you check the fuses in your mystery vehicle?

There can only be a few reasons

Bulbs
The Brake Light Switch
Fuses
Broken Wiring and grounds in the trunk

..

Check the display. Ask a helper to step on the brake pedal while you check the lights from the rear. If no lights appear, check the fuse that connects the brake lights. The owners manual will show the location of the fuse box and which fuse is for the brake lights. Pull out the fuse and look at the wire in the central window. If the wire is broken, replace the fuse. Make sure the new fuse is not blown, and carries the same amperage.
Take note of lights that are out, if any are working. The problem may be cured by replacing the bulb in a brake light that's out. A burned out bulb has a broken filament and a dark brown spot on one side. Check the owners manual for bulb type.
Check the circuit if replacement bulbs and fuses do not restore the lights. Find the wiring diagram in the vehicle assembly manual. The circuit for the brake lights runs from the battery to the fuse, to the pedal switch, to a connection plug, to the lights. The circuit has to be checked with a volt tester at all points.
Check the battery power and the light on your volt tester. Turn the ignition key far enough to activate the electrical systems. Turn on the lights to check for battery power. If the battery has power, ues it to check the volt tester. Put the black lead (the ground) from the volt tester on the negative battery terminal. Briefly touch the red tester light on the positive terminal. The light on the volt tester should flash brightly.
Test the wiring to the fuse. Clip the ground wire to solid metal near the fuse. Touch the light tip to both sides of the fuse. Both should light the tester. If neither side lights, the wire between the battery and fuse is bad. If only one side lights, the fuse is blown.
Test the switch and wiring. Touch the light tip of the tester to both terminals on the brake pedal switch without putting any pressure on the pedal. One should carry power and the other should not. When both show power, the switch is stuck and the brake lights are always on. If neither shows power, the wiring between the fuse and the switch is bad. Now check both switch terminals while the pedal is depressed. Both sides should light the tester. If not, the switch is bad.
Check the connection plug. Unplug the connection and refer to the vehicle manual to identify the brake light wire. Touch the tester light on the brake light wire at the inner socket on the battery side. If the tester lights, the power is flowing to this point. If not, the wire between the plug and the pedal switch is bad.
Check the socket. Plug in the connector plug and remove the bulb. Touch the contact inside the socket. If the tester lights, the socket is good. The socket and wire running to the plug should be replaced if the tester doesn't light.
Check the bulb with the ground wire. The bulb must be in its socket. The ground wire running to the socket should be black or brown. Stab the tester through the insulation. If the tester lights, the bulb is good. Replace the bulb if it doesn't light.
Check the ground wire. Remove some of the insulation around the ground wire. Twist one end of the extra wire around the exposed ground wire. Touch the other end to a solid piece of metal. If the brake light comes on, the ground wire is bad.
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It sounds like you might have a broken wire somewhere if you have no back brake lights.

If any light lights up when you apply brakes, then the brake switch is ok. The problem could be a fuse and/or a relay. or the bulbs. Those bulbs are dual filament so one filament could be bad, for stop or tail lights.

First check for blown fuse. If Ok check bulb, If ok use test light (computer safe led type)
Get someone to put on brake and earth test light lead to bold on vehicle and hold test light probe in centre of bulb holder. It should light up.
If not move earth lead to another bolt on vehcile and retest.
If light comes on in test light the bulb is broken.
If no light in test light the brake light switch connected to the brake pedal is broken. replace brake light switch.

It could be one of three things, the bulbs could have burnt out only for the brake filament...replace a bulb and check.
There is a sensor under the brake pedal that turns those lights on..that could be broken, it looks like a button.
Could also be a fuse, but thats the least likely.

Here are some things to check:-

1 The brake light switch
2 The fuse for the brake lights
3 The bulbs for the brake lights
4 The bulb contacts in the brake lights
5 The earth connections in the rear light clusters
6 Generally check for loose or broken wires in the vucinity of the brake light switch and the rear light clusters.

check the bulbs, check the connector ,check ground and voltage power check fuses and check brake light switch right above the brake padal.

The high mount brake light has its own fuse, so check that first. The high mount bulb is generally a stabdard type (there may be more than one bulb) or you can replace with an LED. As for the left rear bulbs not working when depressing brake pedal, check the bulbs again for any spring that may be broken. If the bulbs are still good, make sure there is sufficient contact with the socket. If still not working, have someone check it out further.

If the driver's side tail light is working, it's not a fuse. Use a test light to check for power at the bulb holder. One terminal will be for tail light, the other for brake/turn light. If the is constant power for tail light, replace bulb. If no power, trace wiring from bulb holder to driver's side junction. Probable broken wire between the two tail lights.