What is the wiring diagram for the various lights in the rear assembly

www.GMTNation.com for lots of Trailblazer into and help

Find where the ground wire for that left taillight grounds to the body remove the screw and clean that ground, otherwise if the taillight assembly uses a printed circuit could be a break in the printing, replace the whole assembly if thats whats going on.

This may sound confusing, so here goes.
Normally I would say check the turn signal switch because for decades the brake light switch power went thru the turn signal switch.
But in your case, its more complicated.
The wiring diagram looks like you have a wire from the brake switch and another from the turn signal that joins inside the right and left rear tail lamp assembly. There are also fuses in the rear fuse box including one for just the brake lights. So its possible that fuse is dead and yet the turn signals would work.
You may have to get a book with the wiring diagrams to trace the circuits.

The AutoZone website may have a wiring diagram in the sites self help repair section.

If you have a voltmeter check for voltage and ground continuity at the bulbs. Check for voltage before and after the brake switch (operate the switch). If you don't have a voltmeter connect something up with a 12V bulb.

If you have voltage after the switch (when operated) but none at the bulb might check the brake/tail light sensor. This is the actual sensor box located in the trunk, drivers side. The sensor can fail resulting no brake lights.

There have been various posts at this forum on the sensor, suggest you do a site search to find these and review them.

But first start checking for voltage.

Auto parts stores have manuals made explicitly for your make and model. It will have the diagram with the color code for all the wires you need.

Use a voltmeter to determine which is the hot wire

Remove one of the rear lights and tap off the + wire on the bottom light wire,there is a opening there to run the wire to the camera.

1996 Geo Tracker Car Radio Wiring Diagram
Car Radio Battery Constant 12v+ Wire: White
Car Radio Accessory Switched 12v+ Wire: White/Black
Car Radio Ground Wire: Black
Car Radio Illumination Wire: Red/Yellow
Car Stereo Dimmer Wire: Red/Green
Car Stereo Antenna Trigger Wire: N/A
Car Stereo Amp Trigger Wire: N/A
Car Stereo Amplifier Location: N/A
Car Audio Front Speakers Size: N/A
Car Audio Front Speakers Location: N/A
Left Front Speaker Positive Wire (+): White
Left Front Speaker Negative Wire (-): Black/Blue
Right Front Speaker Positive Wire (+): Blue
Right Front Speaker Negative Wire (-): White/Red
Car Audio Rear Speakers Size: N/A
Car Audio Rear Speakers Location: N/A
Left Rear Speaker Positive Wire (+): Light Green
Left Rear Speaker Negative Wire (-): Light Green/Black
Right Rear Speaker Positive Wire (+): Gray
Right Rear Speaker Negative Wire (-): Gray/Black

I know this might be a dumb question but, do you have the manual? The Focus owner's manual contains information on how to replace various secondary outside and interior lights.
When you open the hatchback look behind where the light is and you will see a small hole. Unscrew the screw that it is there. You also need to unscrew another screw that is beside the light on the outside (top right). Then you just pull the whole light assembly off the car. Next disconnect the wire from the back of the light bulb assembly. Next, twist the light bulb assemble and pull it out. You now have access to the bulb. Hope that helps.

Go to www.hmaservice.com and register( Vehicle by VIN). Afterward, you have access to shop manuals, service bulletins, wiring diagrams, etc. about your vehicle.